machine: add support for the X9 Pro smartwatch

This is a Chinese smart watch sold on AliExpress etc. It is based on a
nrf52832. More information:
https://github.com/curtpw/nRF5x-device-reverse-engineering/tree/master/X9-nrf52832-activity-tracker

src/machine/board_x9pro.go

@@ -0,0 +1,28 @@
+// +build x9pro
+
+package machine
+
+// https://hackaday.io/project/144350-hacking-wearables-for-mental-health-and-more/details
+const (
+	LED           Pin = 4 // HR LED pin
+	UART_TX_PIN   Pin = NoPin
+	UART_RX_PIN   Pin = NoPin
+	SCL_PIN       Pin = NoPin
+	SDA_PIN       Pin = NoPin
+	SPI0_SCK_PIN  Pin = 18
+	SPI0_MISO_PIN Pin = 19
+	SPI0_MOSI_PIN Pin = 20
+)
+
+// LCD pins.
+const (
+	OLED_CS      Pin = 15 // chip select
+	OLED_RES     Pin = 14 // reset pin
+	OLED_DC      Pin = 13 // data/command
+	OLED_SCK     Pin = 12 // SPI clock
+	OLED_MOSI    Pin = 11 // SPI MOSI (master-out, slave-in)
+	OLED_LED_POW Pin = 16
+	OLED_IC_POW  Pin = 17
+)
+
+const HasLowFrequencyCrystal = true