[MTD] maps/ixp4xx: remove platform specific bits

- Intel chip driver has a reboot notifier so no need to reset the chip here.
- Don't play with chip selects (platform code should do this if necessary).

Signed-off-by: David Vrabel <dvrabel@arcom.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>