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: 621ba4d ("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>

Author: ziyao233

drivers/clk/rockchip/clk.c

@@ -622,7 +622,7 @@ void rockchip_clk_register_branches(struct rockchip_clk_provider *ctx,
 			clk = rockchip_clk_register_mmc(
 				list->name,
 				list->parent_names, list->num_parents,
-				0,
+				NULL,
 				grf, list->muxdiv_offset,
 				list->div_shift
 			);