HAL_ChibiOS: adjust external bus clock for mpu9250

this fixes an issue with fast sampling on the PH2.1 cube

libraries/AP_HAL_ChibiOS/hwdef/fmuv3/hwdef.dat

@@ -132,7 +132,7 @@ SPIDEV ms5611         SPI1 DEVID3  BARO_CS      MODE3 20*MHZ 20*MHZ
 SPIDEV ms5611_ext     SPI4 DEVID2  BARO_EXT_CS  MODE3 20*MHZ 20*MHZ
 SPIDEV mpu6000        SPI1 DEVID4  MPU_CS       MODE3  1*MHZ  8*MHZ 
 SPIDEV mpu9250        SPI1 DEVID4  MPU_CS       MODE3  1*MHZ  8*MHZ 
-SPIDEV mpu9250_ext    SPI4 DEVID1  MPU_EXT_CS   MODE3  1*MHZ  8*MHZ
+SPIDEV mpu9250_ext    SPI4 DEVID1  MPU_EXT_CS   MODE3  4*MHZ  4*MHZ
 SPIDEV hmc5843        SPI1 DEVID5  MAG_CS       MODE3 11*MHZ 11*MHZ
 SPIDEV lsm9ds0_g      SPI1 DEVID4  GYRO_EXT_CS  MODE3 11*MHZ 11*MHZ 
 SPIDEV lsm9ds0_am     SPI1 DEVID3  ACCEL_EXT_CS MODE3 11*MHZ 11*MHZ 
@@ -144,6 +144,14 @@ SPIDEV external0m1    SPI4 DEVID5  MPU_EXT_CS   MODE1  2*MHZ  2*MHZ
 SPIDEV external0m2    SPI4 DEVID5  MPU_EXT_CS   MODE2  2*MHZ  2*MHZ
 SPIDEV external0m3    SPI4 DEVID5  MPU_EXT_CS   MODE3  2*MHZ  2*MHZ
 
+# for SPI clock testing
+#SPIDEV clock500 SPI4 DEVID5  MPU_EXT_CS   MODE0  500*KHZ 500*KHZ # gives 329KHz
+#SPIDEV clock1   SPI4 DEVID5  MPU_EXT_CS   MODE0  1*MHZ 1*MHZ     # gives 657kHz
+#SPIDEV clock2   SPI4 DEVID5  MPU_EXT_CS   MODE0  2*MHZ 2*MHZ     # gives 1.3MHz
+#SPIDEV clock4   SPI4 DEVID5  MPU_EXT_CS   MODE0  4*MHZ 4*MHZ     # gives 2.6MHz
+#SPIDEV clock8   SPI4 DEVID5  MPU_EXT_CS   MODE0  8*MHZ 8*MHZ     # gives 5.5MHz
+#SPIDEV clock16  SPI4 DEVID5  MPU_EXT_CS   MODE0  16*MHZ 16*MHZ   # gives 10.6MHz
+
 define HAL_CHIBIOS_ARCH_FMUV3 1
 
 define HAL_BOARD_LOG_DIRECTORY "/APM/LOGS"