drivers: optee: rpmb: fix returning CID to TEE

The mmc CID value is one of the input parameters used to provision the
RPMB key. The trusted execution environment expects this value to be
specified in big endian format.

Before this fix, on little endian systems, the value returned by the
linux kernel mmc driver differed from the one returned by u-boot.
This meant that if linux provisioned the RPMB key, u-boot would not
have access to the partition (and the other way around).

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>