drivers/memory/spiflash: Rename functions to indicate they use cache.

This patch renames the existing SPI flash API functions to reflect the fact
that the go through the cache:

    mp_spiflash_flush -> mp_spiflash_cache_flush
    mp_spiflash_read  -> mp_spiflash_cached_read
    mp_spiflash_write -> mp_spiflash_cached_write
diff --git a/drivers/memory/spiflash.h b/drivers/memory/spiflash.h
index 66e7906..4837fe3 100644
--- a/drivers/memory/spiflash.h
+++ b/drivers/memory/spiflash.h
@@ -68,8 +68,10 @@
 } mp_spiflash_t;
 
 void mp_spiflash_init(mp_spiflash_t *self);
-void mp_spiflash_flush(mp_spiflash_t *self);
-void mp_spiflash_read(mp_spiflash_t *self, uint32_t addr, size_t len, uint8_t *dest);
-int mp_spiflash_write(mp_spiflash_t *self, uint32_t addr, size_t len, const uint8_t *src);
+
+// These functions use the cache (which must already be configured)
+void mp_spiflash_cache_flush(mp_spiflash_t *self);
+void mp_spiflash_cached_read(mp_spiflash_t *self, uint32_t addr, size_t len, uint8_t *dest);
+int mp_spiflash_cached_write(mp_spiflash_t *self, uint32_t addr, size_t len, const uint8_t *src);
 
 #endif // MICROPY_INCLUDED_DRIVERS_MEMORY_SPIFLASH_H