si5351: add support for si5351 (#810)

* si5351: add support for si5351

Adds support for the si5351 I2C programmable clock generator using code
from @chiefMarlin which used code from @conotto which somehow never got merged.

Thank you everyone!

Signed-off-by: deadprogram <ron@hybridgroup.com>

* refactor: use regmap instead of legacy package to avoid heap allocations

Signed-off-by: deadprogram <ron@hybridgroup.com>

---------

Signed-off-by: deadprogram <ron@hybridgroup.com>

si5351/registers.go

@@ -0,0 +1,64 @@
+package si5351
+
+// The I2C address which this device listens to.
+const AddressDefault = 0x60     // Assumes ADDR pin is low
+const AddressAlternative = 0x61 // Assumes ADDR pin is high
+
+const (
+	OUTPUT_ENABLE_CONTROL = 3
+
+	CLK0_CONTROL = 16
+	CLK1_CONTROL = 17
+	CLK2_CONTROL = 18
+	CLK3_CONTROL = 19
+	CLK4_CONTROL = 20
+	CLK5_CONTROL = 21
+	CLK6_CONTROL = 22
+	CLK7_CONTROL = 23
+
+	MULTISYNTH0_PARAMETERS_1 = 42
+	MULTISYNTH0_PARAMETERS_3 = 44
+	MULTISYNTH1_PARAMETERS_1 = 50
+	MULTISYNTH1_PARAMETERS_3 = 52
+	MULTISYNTH2_PARAMETERS_1 = 58
+	MULTISYNTH2_PARAMETERS_3 = 60
+
+	SPREAD_SPECTRUM_PARAMETERS = 149
+
+	PLL_RESET = 177
+
+	CRYSTAL_INTERNAL_LOAD_CAPACITANCE = 183
+)
+
+const (
+	CRYSTAL_LOAD_6PF  = (1 << 6)
+	CRYSTAL_LOAD_8PF  = (2 << 6)
+	CRYSTAL_LOAD_10PF = (3 << 6)
+)
+
+const (
+	CRYSTAL_FREQ_25MHZ = 25000000
+	CRYSTAL_FREQ_27MHZ = 27000000
+)
+
+const (
+	PLL_A = iota
+	PLL_B
+)
+
+const (
+	R_DIV_1 = iota
+	R_DIV_2
+	R_DIV_4
+	R_DIV_8
+	R_DIV_16
+	R_DIV_32
+	R_DIV_64
+	R_DIV_128
+)
+
+const (
+	MULTISYNTH_DIV_4 = 4
+	MULTISYNTH_DIV_6 = 6
+	MULTISYNTH_DIV_8 = 8
+)