dts/bindings/flash_simulator: Restrict erase-value

The commit restricts erase-value to either 0xff or 0x00.
On program-erase type devices writes can only change from
erase value of bit to opposite, ease usually consists of two
operations which is programming the opposite value to every bit
of erase range and then turning back all bits to the "erase-value";
above makes it impossible to have erase value of device to be other
than 0xff or 0x00.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>