drm: pl111: Credit where credit is due

This moves over some of the credit for the development of this
driver from the old fbdev driver that I used as reference when
getting this in place.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609200446.153209-1-linus.walleij@linaro.org