rockpi4: board: Add firmware image information for capsule updates

Add information that will be needed for enabling the UEFI capsule
update feature on the RockPi4 boards. With the feature enabled, it
would be possible to update the idbloader and u-boot.itb images on the
RockPi4B and RockPi4C variants.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>