simplefb: Include clk.h

This driver uses the consumer API, so include clk.h explicitly
instead of impliclty through the provider API.

Cc: Luc Verhaegen <libv@skynet.be>
Cc: Hans de Goede <hdegoede@redhat.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>