drm/ast: Fix ARM compatibility

ARM architecture only has 'memory', so all devices are accessed by
MMIO if possible.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230421003354.27767-1-jammy_huang@aspeedtech.com