boards: fvp_baser_aemv8r: fix wrong reg size

Some reg size in fvp-aemv8r.dtsi isn't correct. This commit is used to
fix it.

- gic, the correct reg size is 0x10000 and 0x200000.
- uart0-3, the correct reg size is 0x10000.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>