board: arm64: fvp-baser-aemv8r: add all available uart nodes

fvp-baser-aemv8r has four pl011_uart devices and all of then have
been added in this patch.

only uart0 and uart1 are enabled as default in fvp_baser_aemv8r.dts

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