px4_fmu-v6x: Limit Slew rate So IMU works with DMA

The defualt in NuttX is OSPEED of 50Mhz. This is realy a slew
   rate control. At the default high slew rate the overshoot was
   .7 Volts. On a ICM20649 this was causing the device to output
   garbage. All 0s

   N.B. A passive scope or Logic analyser's probes load will mask
   the failure. Useed a FET probe to verify the issue.

boards/px4/fmu-v6x/nuttx-config/include/board.h

@@ -408,25 +408,27 @@
  *
  */
 
-#define GPIO_SPI1_MISO   GPIO_SPI1_MISO_3   /* PG9  */
-#define GPIO_SPI1_MOSI   GPIO_SPI1_MOSI_2   /* PB5  */
-#define GPIO_SPI1_SCK    GPIO_SPI1_SCK_1    /* PA5  */
+#define ADJ_SLEW_RATE(p) (((p) & ~GPIO_SPEED_MASK) | (GPIO_SPEED_2MHz))
 
-#define GPIO_SPI2_MISO   GPIO_SPI2_MISO_3   /* PI2  */
-#define GPIO_SPI2_MOSI   GPIO_SPI2_MOSI_4   /* PI3  */
-#define GPIO_SPI2_SCK    GPIO_SPI2_SCK_6    /* PI1  */
+#define GPIO_SPI1_MISO   GPIO_SPI1_MISO_3               /* PG9  */
+#define GPIO_SPI1_MOSI   GPIO_SPI1_MOSI_2               /* PB5  */
+#define GPIO_SPI1_SCK    ADJ_SLEW_RATE(GPIO_SPI1_SCK_1) /* PA5  */
 
-#define GPIO_SPI3_MISO   GPIO_SPI3_MISO_2   /* PC11 */
-#define GPIO_SPI3_MOSI   GPIO_SPI3_MOSI_3   /* PB2  */
-#define GPIO_SPI3_SCK    GPIO_SPI3_SCK_2    /* PC10 */
+#define GPIO_SPI2_MISO   GPIO_SPI2_MISO_3               /* PI2  */
+#define GPIO_SPI2_MOSI   GPIO_SPI2_MOSI_4               /* PI3  */
+#define GPIO_SPI2_SCK    ADJ_SLEW_RATE(GPIO_SPI2_SCK_6) /* PI1  */
 
-#define GPIO_SPI5_MISO   GPIO_SPI5_MISO_2   /* PH7  */
-#define GPIO_SPI5_MOSI   GPIO_SPI5_MOSI_1   /* PF11 */
-#define GPIO_SPI5_SCK    GPIO_SPI5_SCK_1    /* PF7  */
+#define GPIO_SPI3_MISO   GPIO_SPI3_MISO_2               /* PC11 */
+#define GPIO_SPI3_MOSI   GPIO_SPI3_MOSI_3               /* PB2  */
+#define GPIO_SPI3_SCK    ADJ_SLEW_RATE(GPIO_SPI3_SCK_2) /* PC10 */
 
-#define GPIO_SPI6_MISO   GPIO_SPI6_MISO_2   /* PA6  */
-#define GPIO_SPI6_MOSI   GPIO_SPI6_MOSI_1   /* PG14 */
-#define GPIO_SPI6_SCK    GPIO_SPI6_SCK_3    /* PB3  */
+#define GPIO_SPI5_MISO   GPIO_SPI5_MISO_2               /* PH7  */
+#define GPIO_SPI5_MOSI   GPIO_SPI5_MOSI_1               /* PF11 */
+#define GPIO_SPI5_SCK    ADJ_SLEW_RATE(GPIO_SPI5_SCK_1) /* PF7  */
+
+#define GPIO_SPI6_MISO   GPIO_SPI6_MISO_2               /* PA6  */
+#define GPIO_SPI6_MOSI   GPIO_SPI6_MOSI_1               /* PG14 */
+#define GPIO_SPI6_SCK    ADJ_SLEW_RATE(GPIO_SPI6_SCK_3) /* PB3  */
 
 /* I2C
  *