mediatek: use DEVICE_DTC_FLAGS and drop DTC_FLAGS where not needed

mediatek: use DEVICE_DTC_FLAGS and drop DTC_FLAGS where not needed

The MT7986 RFB was intended to use device tree overlays and for that
reason modified DTC_FLAGS. zyxel_ex5601-t0-stock later on probably
copied it from there. Both boards do not actually use device tree
overlays, so remove setting DTC_FLAGS from both.

The BPi-R3 does use device tree overlays, use DEVICE_DTC_FLAGS to give
it an extra 4kb of padding for overlays to be applied.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 98e6ea32a400b10b425e0efdf5a8109a0dfd48fe)