boards: mRo X2.1-777 fixes

- ardupilot bootloader compatibility (first 3 sectors)
 - clock tree fixes
 - start mavlink
 - start mpu9250 magnetometer (only onboard mag)
 - fixes https://github.com/PX4/Firmware/issues/15466

boards/mro/x21-777/src/timer_config.cpp

@@ -34,7 +34,7 @@
 #include <px4_arch/io_timer_hw_description.h>
 
 constexpr io_timers_t io_timers[MAX_IO_TIMERS] = {
-	initIOTimer(Timer::Timer1),
+	initIOTimer(Timer::Timer1, DMA{DMA::Index2, DMA::Stream5, DMA::Channel6}),
 	initIOTimer(Timer::Timer4),
 };