rp2/mphalport: Fix missing storage_read_blocks symbol in debug build.

When building `make BOARD=PICO_W DEBUG=1` the cyw43-driver requires
storage_read_blocks().

Signed-off-by: Peter Harper <peter.harper@raspberrypi.com>

ports/rp2/mphalport.c

@@ -204,3 +204,8 @@ void mp_hal_get_mac_ascii(int idx, size_t chr_off, size_t chr_len, char *dest) {
         *dest++ = hexchr[mac[chr_off >> 1] >> (4 * (1 - (chr_off & 1))) & 0xf];
     }
 }
+
+// Shouldn't be used, needed by cyw43-driver in debug build.
+uint32_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blocks) {
+    panic_unsupported();
+}