clk: mediatek: clk-mtk: Introduce mtk_clk_get_hwv_regmap()

clk: mediatek: clk-mtk: Introduce mtk_clk_get_hwv_regmap()

On MT8196, some clock controllers use a separate regmap for hardware
voting via set/clear/status registers. Add mtk_clk_get_hwv_regmap() to
retrieve this optional regmap, avoiding duplicated lookup code in 
mtk_clk_register_muxes() and mtk_clk_register_gate().

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>