drm/panthor: Add the devfreq logical block

Every thing related to devfreq in placed in panthor_devfreq.c, and
helpers that can be called by other logical blocks are exposed through
panthor_devfreq.h.

This implementation is loosely based on the panfrost implementation,
the only difference being that we don't count device users, because
the idle/active state will be managed by the scheduler logic.

v6:
- Add Maxime's and Heiko's acks
- Keep header inclusion alphabetically ordered

v4:
- Add Clément's A-b for the relicensing

v3:
- Add acks for the MIT/GPL2 relicensing

v2:
- Added in v2

Cc: Clément Péron <peron.clem@gmail.com> # MIT+GPL2 relicensing
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Steven Price <steven.price@arm.com> # MIT+GPL2 relicensing,Arm
Acked-by: Grant Likely <grant.likely@linaro.org> # MIT+GPL2 relicensing,Linaro
Acked-by: Boris Brezillon <boris.brezillon@collabora.com> # MIT+GPL2 relicensing,Collabora
Acked-by: Clément Péron <peron.clem@gmail.com> # MIT+GPL2 relicensing
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240229162230.2634044-7-boris.brezillon@collabora.com