mt76: mmio: add implementation of wr_rp and rd_rp

Introduce mmio implementation of mt76_mmio_rd_rp and
mt76_mmio_wr_rp and remove mt76x2 duplicated code.
mt76_mmio_{wr,rd}_rp allows to reuse mt76x0 init code
adding mt76x0e support

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>