drm: meson: add macro used to enable HDMI PLL

This patch add new macro HHI_HDMI_PLL_CNTL_EN which is used to enable
HDMI PLL.

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86o92n82e1.fsf@baylibre.com