simplefb: add clock handling code

This claims and enables clocks listed in the simple framebuffer dt node.
This is needed so that the display engine, in case the required clocks
are known by the kernel code and are described in the dt, will remain
properly enabled.

Signed-off-by: Luc Verhaegen <libv@skynet.be>
[hdegoede@redhat.com: Change clks from list to dynamic array]
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>

--
Changes in v4:
-change clks from linkedlist to dynamic allocated array
-propagate EPROBE_DEFER up from simplefb_clocks_init to simplefb_probe