mtd: unify status enum from three headers

nand.h, onenand.h and flashchip.h defined enumeration types
for chip status using the same symbolic names. This prevented
a board file to include more than one of them. In particular,
no nand and onenand platform devices could live in the same file.
This patch augments flashchip.h with a few status values in order
to cover all cases, so nand.h and onenand.h can use flstate_t
without declaring their own status enum.

Signed-off-by: Alessandro Rubini <rubini@unipv.it>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>