platform: kendryte/k210: fixup FDT

When passing the device tree to the next stage we should apply necessary
device tree fix-ups first. These include:

* fix up the CPU node in the device tree
* fix up the PLIC node in the device tree
* fix up the reserved memory node in the device tree

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>