rockchip: use UUID for root partitions

rockchip: use UUID for root partitions

We use to use /dev/mmcbl0p7 as root partition, and pass it
to kernel by cmdline, but the mmc number in kernel in not
fixed, we need to change the bootargs to adapt it from time
to time.
We can use the UUID to fix it, the ID is from:
https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
ARM 32bit: 69dad710-2ce4-4e3c-b16c-21a1d49abed3
ARM 64bit: b921b045-1df0-41c3-af44-4c6f280d3fae

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>