clk: meson: g12a: fix VPU clock muxes mask

There are 8 parents, use 0x7

Fixes: 085a4ea9 ("clk: meson: g12a: add peripheral clock controller")
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/20190319082611.6215-1-mjourdan@baylibre.com