Fix SPI + MPU

[Peque]

MPU is currently not responding for unknown reasons. We should investigate this further.

Also, SPI communications seem to be broken and require some wait time before reading/writing:

diff --git a/src/platform.c b/src/platform.c
index 6776f13..f9472a7 100644
--- a/src/platform.c
+++ b/src/platform.c
@@ -74,6 +74,7 @@ uint8_t mpu_read_register(uint8_t address)
        spi_send(SPI3, 0x00);
        reading = spi_read(SPI3);
        gpio_set(GPIOA, GPIO15);
+       sleep_us(4);
 
        return reading;
 }
@@ -92,6 +93,7 @@ void mpu_write_register(uint8_t address, uint8_t value)
        spi_send(SPI3, value);
        spi_read(SPI3);
        gpio_set(GPIOA, GPIO15);
+       sleep_us(4);
 }
 
 /**

That was not the case with Bulebule. Why did it change? Could we use hardware chip-select?

[Peque]

Hardware chip-select would be enabled in next PCB batch (if ever...): #41