mtd: remove junk pmc551.h

This header is tiny and contains only pmc551-private stuff, so it should
not live in 'include/linux' - let's just merge it with pmc551.c.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>