drm/panthor: Implement reading shader_present from nvmem

drm/panthor: Implement reading shader_present from nvmem

On some platforms, notably MediaTek MT8196, the shader_present bitmask
in the Mali GPU register for it has cores enabled that may be faulty.
The true shader_present bitmask is found in an efuse instead.

Implement reading shader_present from an nvmem cell if one is present,
falling back to the Mali register if it's absent. The error codes are
trickled up through to the probe function so that probe deferral works.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251220-mt8196-shader-present-v2-3-45b1ff1dfab0@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>