MMC: add weak function to detect MMC/SD card

Most controllers can check if there is a card in the slot.
However, they require pins that could be not available because
required by other functions and the detection of a card must
be performed in another way. This patch adds a weak function
that a board can implement to add its internal custom way
to check the presence of a MMC/SD card.
Signed-off-by: NStefano Babic <sbabic@denx.de>

drivers/mmc/mmc.c

@@ -36,6 +36,13 @@
 static struct list_head mmc_devices;
 static int cur_dev_num = -1;
 
+int __board_mmc_getcd(u8 *cd, struct mmc *mmc) {
+	return -1;
+}
+
+int board_mmc_getcd(u8 *cd, struct mmc *mmc)__attribute__((weak,
+	alias("__board_mmc_getcd")));
+
 int mmc_send_cmd(struct mmc *mmc, struct mmc_cmd *cmd, struct mmc_data *data)
 {
 	return mmc->send_cmd(mmc, cmd, data);

include/mmc.h

@@ -272,6 +272,7 @@ int mmc_init(struct mmc *mmc);
 int mmc_read(struct mmc *mmc, u64 src, uchar *dst, int size);
 struct mmc *find_mmc_device(int dev_num);
 void print_mmc_devices(char separator);
+int board_mmc_getcd(u8 *cd, struct mmc *mmc);
 
 #ifndef CONFIG_GENERIC_MMC
 int mmc_legacy_init(int verbose);