mtd: maps: gpio-addr-flash: add error handling for ioremap_nocache

When ioremap_nocache fails, the lack of error-handling code may
cause unexpected results.

This patch adds error-handling code after calling ioremap_nocache.

Signed-off-by: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>