targets: add support for Pimoroni Tufty 2350 board

Add a tufty2350 target and board file for the Pimoroni Tufty 2350,
an RP2350B board with a parallel ST7789 LCD and a CYW43439 wireless
chip. The target inherits rp2350b and sets a 16 MB flash size, like
the existing pga2350 and pico-plus2 targets.

Pin names come from Pimoroni's published pins.csv for the board:
https://github.com/pimoroni/tufty2350/blob/main/board/pins.csv

src/machine/board_tufty2350.go

@@ -0,0 +1,107 @@
+//go:build tufty2350
+
+// This contains the pin mappings for the Pimoroni Tufty 2350 board.
+//
+// For more information, see: https://shop.pimoroni.com/products/tufty-2350
+// Pinout source: https://github.com/pimoroni/tufty2350/blob/main/board/pins.csv
+package machine
+
+// Case LEDs (4-zone mono illumination), named CL0..CL3 in the docs.
+const (
+	CL0 Pin = GPIO0
+	CL1 Pin = GPIO1
+	CL2 Pin = GPIO2
+	CL3 Pin = GPIO3
+
+	// Convention: first zone as default LED.
+	LED = CL0
+)
+
+const (
+	BUTTON_DOWN  Pin = GPIO6
+	BUTTON_A     Pin = GPIO7
+	BUTTON_B     Pin = GPIO9
+	BUTTON_C     Pin = GPIO10
+	BUTTON_UP    Pin = GPIO11
+	BUTTON_HOME  Pin = GPIO22
+	BUTTON_RESET Pin = GPIO14
+	BUTTON_INT   Pin = GPIO15
+
+	VBUS_DETECT Pin = GPIO12
+	RTC_ALARM   Pin = GPIO13
+
+	LCD_BACKLIGHT Pin = GPIO26
+	LCD_CS        Pin = GPIO27
+	LCD_DC        Pin = GPIO28
+	LCD_WR        Pin = GPIO30
+	LCD_RD        Pin = GPIO31
+	LCD_DB0       Pin = GPIO32
+	LCD_DB1       Pin = GPIO33
+	LCD_DB2       Pin = GPIO34
+	LCD_DB3       Pin = GPIO35
+	LCD_DB4       Pin = GPIO36
+	LCD_DB5       Pin = GPIO37
+	LCD_DB6       Pin = GPIO38
+	LCD_DB7       Pin = GPIO39
+
+	VBAT_SENSE  Pin = GPIO40
+	POWER_EN    Pin = GPIO41
+	SENSE_1V1   Pin = GPIO42
+	LIGHT_SENSE Pin = GPIO43
+)
+
+// CYW43439 wireless chip control pins.
+const (
+	WL_REG_ON Pin = GPIO23
+	WL_DATA   Pin = GPIO24
+	WL_CLOCK  Pin = GPIO29
+	WL_CS     Pin = GPIO25
+)
+
+// I2C pins. I2C0 carries the onboard PCF85063 real time clock.
+const (
+	I2C0_SDA_PIN Pin = GPIO4
+	I2C0_SCL_PIN Pin = GPIO5
+
+	I2C1_SDA_PIN Pin = NoPin
+	I2C1_SCL_PIN Pin = NoPin
+)
+
+// SPI pins. No SPI peripheral is wired to a header on this board.
+const (
+	SPI0_SCK_PIN Pin = NoPin
+	SPI0_SDO_PIN Pin = NoPin
+	SPI0_SDI_PIN Pin = NoPin
+
+	SPI1_SCK_PIN Pin = NoPin
+	SPI1_SDO_PIN Pin = NoPin
+	SPI1_SDI_PIN Pin = NoPin
+)
+
+// Onboard crystal oscillator frequency, in MHz.
+const (
+	xoscFreq = 12 // MHz
+)
+
+// USB CDC identifiers
+const (
+	usb_STRING_PRODUCT      = "Tufty 2350"
+	usb_STRING_MANUFACTURER = "Pimoroni"
+)
+
+var (
+	usb_VID uint16 = 0x2e8a
+	usb_PID uint16 = 0x1101
+)
+
+// UART pins.
+// Note: GPIO0/GPIO1 are also CL0/CL1 (case LEDs).
+// Do not use UART0 and the case LEDs at the same time.
+const (
+	UART0_TX_PIN = GPIO0
+	UART0_RX_PIN = GPIO1
+	UART_TX_PIN  = UART0_TX_PIN
+	UART_RX_PIN  = UART0_RX_PIN
+)
+
+var DefaultUART = UART0