clk: mediatek: add drivers for MT7988 SoC

Add APMIXED, ETH, INFRACFG and TOPCKGEN clock drivers which are
typical MediaTek designs.

Also add driver for XFIPLL clock generating the 156.25MHz clock for
the XFI SerDes. It needs an undocumented software workaround and has
an unknown internal design.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/c7574d808e2da1a530182f0fd790c1337c336e1b.1702849494.git.daniel@makrotopia.org
[sboyd@kernel.org: Add module license to infracfg file]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>