mt76: mt7915: introduce __mt7915_get_tsf routine

Introduce an unlocked verion of mt7915_get_tsf routine.
This is a preliminary patch to add TWT support to mt7915.

Tested-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>