[media] s5p-mfc: Add variants to access mfc registers

This patch is needed in preparation to add MFC V8
where the register offsets are changed w.r.t MFC V6/V7.

This patch adds variants of MFC V6 and V7 while
accessing MFC registers. Registers are kept in mfc context
and are initialized to a particular MFC variant during probe,
which is used instead of macros.

This avoids duplication of the code for MFC variants
V6 & V7, and reduces the if_else checks while accessing
registers of different MFC variants.

Signed-off-by: Kiran AVND <avnd.kiran@samsung.com>
Signed-off-by: Pawel Osciak <posciak@chromium.org>
Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>