clk: meson-g12a: add video decoder clocks

Add the necessary clock parts for:

 - VDEC_1: used to feed VDEC_1
 - VDEC_HEVC: the "back" part of the VDEC_HEVC block
 - VDEC_HEVCF: the "front" part of the VDEC_HEVC block

In previous SoC generations (GXL, GXBB), there was only one VDEC_HEVC
clock, which got split in two parts for G12A.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20190319101138.27520-2-mjourdan@baylibre.com