[MTD] mtdchar.c: Replace DEVFS by udev

Switch from DEVFS to udev for dynamic creation of device nodes for mtd
char devices.

Creates a new LDM class "mtd" with writeable and read-only devices
registered for each mtdchar device.

From: Paolo Galtieri <pgaltieri@mvista.com>
Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>