reset: meson: add level reset support for GX SoC family

The Amlogic GX SoC family embeds alternate registers to drive the reset
levels next to the pulse registers.

This patch adds support for level reset handling on the GX family only.

The Meson8 family has an alternate way to handle level reset.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>