stm32/boards/STM32L476DISC: Update SPI flash config for cache change.

ports/stm32/boards/STM32L476DISC/bdev.c

@@ -2,7 +2,7 @@
 
 // External SPI flash uses standard SPI interface
 
-const mp_soft_spi_obj_t soft_spi_bus = {
+STATIC const mp_soft_spi_obj_t soft_spi_bus = {
     .delay_half = MICROPY_HW_SOFTSPI_MIN_DELAY,
     .polarity = 0,
     .phase = 0,
@@ -11,11 +11,14 @@ const mp_soft_spi_obj_t soft_spi_bus = {
     .miso = MICROPY_HW_SPIFLASH_MISO,
 };
 
+STATIC mp_spiflash_cache_t spi_bdev_cache;
+
 const mp_spiflash_config_t spiflash_config = {
     .bus_kind = MP_SPIFLASH_BUS_SPI,
     .bus.u_spi.cs = MICROPY_HW_SPIFLASH_CS,
     .bus.u_spi.data = (void*)&soft_spi_bus,
     .bus.u_spi.proto = &mp_soft_spi_proto,
+    .cache = &spi_bdev_cache,
 };
 
 spi_bdev_t spi_bdev;