media: s5p-mfc: Make additional clocks optional

Since the beginning the second clock ('special', 'sclk') was optional and
it is not available on some variants of Exynos SoCs (i.e. Exynos5420 with
v7 of MFC hardware).

However commit 1bce6fb3 ("[media] s5p-mfc: Rework clock handling")
made handling of all specified clocks mandatory. This patch restores
original behavior of the driver and fixes its operation on
Exynos5420 SoCs.

Fixes: 1bce6fb3

 ("[media] s5p-mfc: Rework clock handling")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>