drivers:optee:rpmb: initialize drivers of mmc devices in UCLASS_BLK for rpmb access

CONFIG_MMC only initializes drivers for devices in UCLASS_MMC, we need
to initialize drivers for devices of type IF_TYPE_MMC in UCLASS_BLK as
well because they are the child devices of devices in UCLASS_MMC.  This
is required for feature RPMB since it will access eMMC in optee-os.

Signed-off-by: Judy Wang <wangjudy@microsoft.com>
[trini: Add my SoB line and adjust Judy's name in git, having emailed
off-list]
Signed-off-by: Tom Rini <trini@konsulko.com>