drm: renesas: shmobile: Rename shmob_drm_crtc.crtc

Rename the "crtc" member of the shmob_drm_crtc subclass structure to
"base", to improve readability.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/b42a32082d25bde6db7a57d8dc0d9f45820f6716.1694767209.git.geert+renesas@glider.be