[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.