MAINTAINERS: Document Imagination PowerVR driver patches go via drm-misc

This is the tree used by nearly all other DRM drivers, so use it for the
PowerVR driver as well.

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231204132847.1307340-1-frank.binns@imgtec.com