HAL_ChibiOS: enable clock management on G4

this allows for a different clock tree in the main fw from the
bootloader

libraries/AP_HAL_ChibiOS/CANFDIface.cpp

@@ -89,7 +89,9 @@ extern const AP_HAL::HAL& hal;
 
 #define STR(x) #x
 #define XSTR(x) STR(x)
+#if !defined(HAL_LLD_USE_CLOCK_MANAGEMENT)
 static_assert(STM32_FDCANCLK == 80U*1000U*1000U, "FDCAN clock must be 80MHz, got " XSTR(STM32_FDCANCLK));
+#endif
 
 using namespace ChibiOS;
 

libraries/AP_HAL_ChibiOS/hwdef/common/stm32g4_mcuconf.h

@@ -25,6 +25,12 @@
 #define STM32_LSEDRV                (3U << 3U)
 #endif
 
+/*
+  we need to use HAL_LLD_USE_CLOCK_MANAGEMENT in order to allow a
+  different clock tree in the main firmware from the bootloader.
+ */
+#define HAL_LLD_USE_CLOCK_MANAGEMENT 1
+
 /*
  * STM32G4xx drivers configuration.
  * The following settings override the default settings present in