Add generic F412Cx pinout variant

[MCUdude]

This pinout variant is more or less identical with F401Cx, F410Cx, and F411Cx, but (of course) designed to work with the F412Cx series.

[fpistm]

Small Astyle fix:

--- a/variants/Generic_F412Cx/PeripheralPins.c
+++ b/variants/Generic_F412Cx/PeripheralPins.c
@@ -84,7 +84,7 @@ WEAK const PinMap PinMap_PWM[] = {
   //  {PA_3,  TIM9,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF3_TIM9, 2, 0)}, // TIM9_CH2
   {PA_5,  TIM2,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF1_TIM2, 1, 0)}, // TIM2_CH1
   //  {PA_5,  TIM8,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF3_TIM8, 1, 1)}, // TIM8_CH1N
-   //  {PA_6,  TIM3,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF2_TIM3, 1, 0)}, // TIM3_CH1
+  //  {PA_6,  TIM3,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF2_TIM3, 1, 0)}, // TIM3_CH1
   {PA_6,  TIM13,  STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF9_TIM13, 1, 0)}, // TIM13_CH1
   //  {PA_7,  TIM1,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF1_TIM1, 1, 1)}, // TIM1_CH1N
   //  {PA_7,  TIM3,   STM_PIN_DATA_EXT(STM_MODE_AF_PP, GPIO_PULLUP, GPIO_AF2_TIM3, 2, 0)}, // TIM3_CH2

To avoid this you can refer to:
https://github.com/stm32duino/wiki/wiki/Astyle