mt76: mt7915: add bf backoff limit table support

mt76: mt7915: add bf backoff limit table support

mt76 as support for generic rates power limits in the devicetree. But the
mt7915 supports beamforming and has another table for configuring the
backoff limits. These can be configured in the DT with the paths-*
properties. The path-*-bf are the ones relevant for beamforming and the
ones without -bf suffix for "traditional" path backoff.

Signed-off-by: Sven Eckelmann (Plasma Cloud) <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20152
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>