clk: rockchip: Pass NULL as reg pointer when registering GRF MMC clocks

clk: rockchip: Pass NULL as reg pointer when registering GRF MMC clocks

This corrects the type and suppresses sparse warnings about passing
plain integers as NULL pointer.

Fixes: 621ba4d9f6db ("clk: rockchip: Support MMC clocks in GRF region")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505100302.YVtB1zhF-lkp@intel.com/
Signed-off-by: Yao Zi <ziyao@disroot.org>
Link: https://lore.kernel.org/r/20250510075248.34006-2-ziyao@disroot.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>