mediatek: dts: bring mt7988a.dtsi closer to upstream

mediatek: dts: bring mt7988a.dtsi closer to upstream

In preparation of using the upstream mt7988a.dtsi when switching
to Linux 6.12 prepare by bringing our downstream version closer to
what went upstream.
 * rename 'xphy' -> 'xsphy'
 * rename 'uart[012]' -> 'serial[012]'
 * only list pinctrl settings directly used in mt7988a.dtsi there,
   leave it to boards to define all additional pinctrl settings
   they need.
 * move fan and thermal-zone to board level

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