mtd: nand: diskonchip: Request memory region prior ioremap()

This patch adds request_mem_region() prior ioremap() for diskonchip
driver. This will allow to check if memory region is occupied by any
other device, for example in case if we have memory region for several
optional devices and only one device can be used at once.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>