ARM64: dts: mediatek: add pericfg syscon to mt8516.dtsi

This adds support for the PERICFG register range as a syscon. This will
soon be used by the MediaTek Ethernet MAC driver for NIC configuration.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>