mpc85xx: use gpio-button-hotplug as input subsystem

mpc85xx: use gpio-button-hotplug as input subsystem

This replaces the standard Linux input subsystem with the
gpio-button-hotplug driver, which is already used with most other
targets for over a decade. While reducing overhead and resource
pressure especially for resource-constrained devices, this will
also allow for configuring things like flip-switches, in addition
to standard pushbuttons.

Will be required for a following commit.

Signed-off-by: Shine <4c.fce2@proton.me>
Tested-by: Edward Chow <equu@openmail.cc>
Link: https://github.com/openwrt/openwrt/pull/22494
Signed-off-by: Robert Marko <robimarko@gmail.com>