generic: 6.12: mtk_eth_soc: adapt downstream SerDes patch

generic: 6.12: mtk_eth_soc: adapt downstream SerDes patch

While it will hopefully be replaced soon by an upstream solution for
now we keep the downstream SerDes PCS implementation for MT7988A.
In order to make it work with upstream mt7988a.dtsi we have to
compensate for the changed start address of topmisc which was
moved from 0x11d10000 to 0x11d10084 to accomodate a future power
domain controller located at 0x11d10000~0x11d10080.
Hence we need to change TOP_MISC_NETSYS_PCS_MUX from 0x84 to 0x0, so
the Ethernet path/mux selection can work with upstream mt7988a.dtsi.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>