clk: meson: meson8b: add the vclk2_en gate clock

HHI_VIID_CLK_CNTL[19] is not part of the public S805 datasheet. However,
the GXBB driver defines this bit as a gate called "vclk2" and in the
3.10 kernel GPL code dump the following line can found:
  WRITE_LCD_CBUS_REG_BITS(HHI_VIID_CLK_CNTL, 0, 19, 1); //disable vclk2_en

Add this gate clock to the Meson8/Meson8b/Meson8m2 clock controller to
complete the VCLK2 clock tree.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200629203904.2989007-3-martin.blumenstingl@googlemail.com