stm32: Support PA11/PA12 pin remap on stm32f042 tsop20 chip

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32/Kconfig

@@ -124,6 +124,10 @@ config USBSERIAL
     bool "Use USB for communication (instead of serial)"
     depends on HAVE_STM32_USBFS || HAVE_STM32_USBOTG
     default y
+config STM32F042_USB_PIN_SWAP
+    bool "Use PA9/PA10 for USB" if MACH_STM32F042
+    depends on USBSERIAL && MACH_STM32F042
+    default n
 config SERIAL
     depends on !USBSERIAL
     bool

src/stm32/stm32f0.c

@@ -159,4 +159,12 @@ clock_setup(void)
         hsi48_setup();
     else
         pll_setup();
+
+    // Support alternate USB pins on stm32f042
+#ifdef SYSCFG_CFGR1_PA11_PA12_RMP
+    if (CONFIG_STM32F042_USB_PIN_SWAP) {
+        enable_pclock(SYSCFG_BASE);
+        SYSCFG->CFGR1 |= SYSCFG_CFGR1_PA11_PA12_RMP;
+    }
+#endif
 }