rockchip: puma-rk3399: do not hardcode MMC controller paths

To prepare to put the similar logic around storage medium selection for
Ringneck PX30 and Puma RK3399 in common, let's not use hardcoded paths
but use uclass functions instead to find udevice based on their DT node
full path.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>