clk: zte: add pll_vga clock for zx296718

It adds zx296718 pll_vga clock for VGA support, so that VGA device can
get required pixel rate from clock driver for different display mode.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>