clk: rockchip: add helper to register auxiliary GRFs

clk: rockchip: add helper to register auxiliary GRFs

Add rockchip_clk_add_grf() as a helper to register an auxiliary GRF
into the clock provider's aux_grf_table. This encapsulates the
struct rockchip_aux_grf allocation and hashtable insertion, so SoC
clock drivers do not need to open-code it.

Suggested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Daniele Briguglio <hello@superkali.me>
Link: https://patch.msgid.link/20260419-rk3588-mclk-gate-grf-v4-3-513a42dd1dcc@superkali.me
Signed-off-by: Heiko Stuebner <heiko@sntech.de>