rockchip: video: Kconfig: set MAX_XRES and MAX_YRES via Kconfig

This introduces two new Kconfig options that configure the maximum
allowable framebuffer size (i.e. the memory reservation/allocation for
the framebuffer):
 - VIDEO_ROCKCHIP_MAX_XRES
 - VIDEO_ROCKCHIP_MAX_YRES
The resulting memory allocation will cover 4 byte per pixel for these
resolutions.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>