m25p80: add support for two bytes WRSR for Macronix chips

On Macronix chips, two bytes can written to the WRSR. First byte will
configure the status register and the second the configuration
register. It is important to save the configuration value as it
contains the dummy cycle setting when using dual or quad IO mode.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>