clk: mediatek: Ensure fhctl code is available for COMMON_CLK_MT6795

Without this select we get linker errors when linking
clk-mt6795-apmixedsys

arm-linux-gnueabi-ld: drivers/clk/mediatek/clk-mt6795-apmixedsys.o: in function `clk_mt6795_apmixed_remove':
clk-mt6795-apmixedsys.c:(.text+0x34): undefined reference to `mtk_clk_unregister_pllfhs'
arm-linux-gnueabi-ld: drivers/clk/mediatek/clk-mt6795-apmixedsys.o: in function `clk_mt6795_apmixed_probe':
clk-mt6795-apmixedsys.c:(.text+0x98): undefined reference to `fhctl_parse_dt'
arm-linux-gnueabi-ld: clk-mt6795-apmixedsys.c:(.text+0xb8): undefined reference to `mtk_clk_register_pllfhs'
arm-linux-gnueabi-ld: clk-mt6795-apmixedsys.c:(.text+0x1c4): undefined reference to `mtk_clk_unregister_pllfhs'

Fixes: f222a1ba ("clk: mediatek: mt6795: Add support for frequency hopping through FHCTL")
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20230316231118.2579242-1-sboyd@kernel.org