mtd: m25p80: Flash protection support for STmicro chips

This patch adds generic support for flash protection on STmicro chips.
On chips with less than 3 protection bits, the unused bits are don't cares
and so can be written anyway. The lock function will only change the
protection bits if it would not unlock other areas. Similarly, the unlock
function will not lock currently unlocked areas. Tested on the m25p64.

Signed-off-by: Austin Boyle <Austin.Boyle@aviatnet.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>