drivers: flash: spi_nor: Added flag status register support

Some Micron (and possibly other) SPI NOR devices implement a flag status
register which provides more information on the success/failure of erase
and program operations. In addition to better error checking, some of
these devices actually don't function properly if the flag status
register is not read after a program operation (subsequent reads will
only return 0xFF bytes).

Add a device tree parameter to indicate that the flag status register is
supported. When specified, the flag status register will be used for
ready/error checks rather than the standard status register.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>