I want to sort out support for tinydrm in vc4, so I needed to get a
tinydrm-appropriate panel working and this is what I had on hand.
This is derived from a combination of ili9341.c from tinydrm and
fb_hx8357d.c from staging's fbtft.

v2: Write my own register defs from the spec to not need the header
    from fbtft.  Fix spi device string to enable module autoloading.
    (Suggestions by Noralf)
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024184313.2967-3-eric@anholt.net
Reviewed-by: Noralf Trønnes <noralf@tronnes.org> (v1)