drm/meson: add mode selection limits against specific SoC revisions

The Amlogic S805X/Y uses the same die as the S905X, but with more
limited graphics capabilities.

This adds a soc version detection adding specific limitations on the HDMI
mode selections.

Here, we limit to HDMI 1.2a max HDMI PHY clock frequency.

Changes sinces v1:
- Moved frequency check in the vclk code, and also checks DMT modes

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
[narmstrong: fixed commit message with HDMI 1.2a instead of HDMI 1.3a]
Link: https://patchwork.freedesktop.org/patch/msgid/20200428092147.13698-1-narmstrong@baylibre.com