feat(mmc): get boot partition size

The boot partition size of an eMMC is given in ext_csd register, at
offset 226 (BOOT_SIZE_MULT), which has to be multiplied by 128kB.
Add a helper function mmc_boot_part_size() to get this eMMC boot
partition size.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0e8e0fc9632f147fa1b1b3374accb78439025403
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index 1b6e96b..51e94be 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -805,6 +805,11 @@
 	return ret;
 }
 
+size_t mmc_boot_part_size(void)
+{
+	return mmc_ext_csd[CMD_EXTCSD_BOOT_SIZE_MULT] * SZ_128K;
+}
+
 size_t mmc_boot_part_read_blocks(int lba, uintptr_t buf, size_t size)
 {
 	size_t size_read;