i2c: mt65xx: Simplify with clk-bulk

Since depending on the SoC or specific bus functionality some clocks
may be optional, we cannot get the benefit of using devm_clk_bulk_get()
but, by migrating to clk-bulk, we are able to remove the custom functions
mtk_i2c_clock_enable() and mtk_i2c_clock_disable(), increasing common
APIs usage, hence (lightly) decreasing kernel footprint.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>