mtd: m25p80: restore the status of SPI flash when exiting

Restore the status to be compatible with legacy devices.
Take Freescale eSPI boot for example, it copies (in 3 Byte
addressing mode) the RCW and bootloader images from SPI flash
without firing a reset signal previously, so the reboot command
will fail without resetting the addressing mode of SPI flash.
This patch implements .shutdown function to restore the status
in reboot process, and add the same operation to the .remove
function.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>