mmc: adjust Kconfig for mmc sub-commands

All sub-commands of the mmc command should be shown in the Kconfig menu
next to the mmc command. This includes:

* mmc bkops
* mmc rpmb
* mmc swrite

The mmc rpmb sub-command is not usable without CONFIG_SUPPORT_EMMC_RPMB.
Add the missing dependency.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>