drivers: mmc: rpmb: do not build for SPL

RPMB support is used by the 'mmc rpmb' command and by the OP-TEE support.
We do not need it in SPL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>