clk: meson-gxbb: Add GXL/GXM GP0 Variant

The clock tree in the Amlogic GXBB and GXL/GXM SoCs is shared, but the GXL/GXM
SoCs embeds a different GP0 PLL, and needs different parameters with a vendor
provided reduced rate table.

This patch adds the GXL GP0 variant, and adds a GXL DT compatible in order
to use the GXL GP0 PLL instead of the GXBB specific one.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1490178747-14837-4-git-send-email-narmstrong@baylibre.com