Convert CONFIG_SUPPORT_EMMC_RPMB to Kconfig

Convert CONFIG_SUPPORT_EMMC_RPMB to Kconfig. Split the command handling
from the underlying support and expose this through CMD_MMC_RPMB.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index 3f15f85..693b3ce 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -71,6 +71,13 @@
 	  This adds a command and an API to do hardware partitioning on eMMC
 	  devices.
 
+config SUPPORT_EMMC_RPMB
+	bool "Support eMMC replay protected memory block (RPMB)"
+	imply CMD_MMC_RPMB
+	help
+	  Enable support for reading, writing and programming the
+	  key for the Replay Protection Memory Block partition in eMMC.
+
 config MMC_IO_VOLTAGE
 	bool "Support IO voltage configuration"
 	help