mtd: bcm47xxsflash: define opcodes

We need them to add erase/write support. This may duplicate some defines
with bcma and/or ssb code, but it makes more sense to keep that in
bcm47xxsflash which is supposed to work with both buses.
Duplicated defines will be removed from ssb/bcma.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>