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

drivers/gpu/drm/pl111/pl111_versatile.c

 // SPDX-License-Identifier: GPL-2.0-only
 
+/*
+ * Versatile family (ARM reference designs) handling for the PL11x.
+ * This is based on code and know-how in the previous frame buffer
+ * driver in drivers/video/fbdev/amba-clcd.c:
+ * Copyright (C) 2001 ARM Limited, by David A Rusling
+ * Updated to 2.5 by Deep Blue Solutions Ltd.
+ * Major contributions and discoveries by Russell King.
+ */
+
 #include <linux/amba/clcd-regs.h>
 #include <linux/bitops.h>
 #include <linux/device.h>