cfi_flash: add support for Spansion flash PPB sector protection

Erasing flash sectors protected with persistent protection bit (PPB)
mechanism on Spansion flash chips doesn't work. Add sector protection
status checking and sector lock and unlock commands to fix this.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>