drm: Add simpledrm driver

The simpledrm driver is a DRM driver for simple-framebuffer framebuffers
as provided by the kernel's boot code. This driver enables basic
graphical output on many different graphics devices that are provided
by the platform (e.g., EFI, VESA, embedded framebuffers).

With the kernel's simple-framebuffer infrastructure, the kernel receives
a pre-configured framebuffer from the system (i.e., firmware, boot
loader). It creates a platform device to which simpledrm attaches.
The system's framebuffer consists of a memory range, size and format.
Based on these values, simpledrm creates a DRM devices. No actual
modesetting is possible.

A firmware framebuffer might also be specified via device-tree files. If
no device platform data is given, try the DT device node.

Make sure required hardware clocks and regulators are enabled while the
firmware framebuffer is in use. The basic code has been taken from the
simplefb driver and adapted to DRM. Clocks are released automatically
via devres helpers.

The drivers displays a console on simpledrm's framebuffer. The default
framebuffer format is being used.

v4:
	* disable simplefb if simpledrm has been selected (Maxime)
v3:
	* add disable function that clears screen to black (Daniel)
	* set shadow buffering only for fbdev emulation
	* set platform-driver data during device creation
v2:
	* rename driver to simpledrm
	* add dri-devel to MAINTAINERS entry
	* put native format first in primary-plane format list (Daniel)
	* inline simplekms_device_cleanup() (Daniel)
	* use helpers for shadow-buffered planes
	* fix whitespace errors
	* add Device Tree match table
	* clean-up parser wrappers
	* use strscpy()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> # fbdev support
Acked-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-5-tzimmermann@suse.de
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-6-tzimmermann@suse.de
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-7-tzimmermann@suse.de
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-8-tzimmermann@suse.de
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-9-tzimmermann@suse.de