clk: bcm: rpi: Add register to control pixel bvb clk

clk: bcm: rpi: Add register to control pixel bvb clk

To use QHD or higher, we need to modify the pixel_bvb_clk value. So
add register to control this clock.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Link: https://lore.kernel.org/r/20200901040759.29992-2-hoegeun.kwon@samsung.com
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>