drm: pl111: Update documentation

Remove notes about migrating from the old driver which is
retired as all users are now migrated.

Update the text to reflect that we support PL110 and PL111
alike.

Drop the bullet on memory bandwidth scaling: this has been
implemented.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720130327.92364-1-linus.walleij@linaro.org