rockchip: add weak function symbol called at the beginning of misc_init_r

rockchip: add weak function symbol called at the beginning of misc_init_r

Most Rockchip boards who override misc_init_r do it only to call another
function and keep the rest unchanged. Therefore to allow for less
duplication, let's just add a weak function symbol that is called inside
misc_init_r.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>