arc: iounmap() arg is volatile

Add 'volatile' to iounmap()'s argument to prevent build warnings.
This make it the same as other major architectures.

Placates these warnings: (12 such warnings)

../drivers/video/fbdev/riva/fbdev.c: In function 'rivafb_probe':
../drivers/video/fbdev/riva/fbdev.c:2067:42: error: passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type [-Werror=discarded-qualifiers]
 2067 |                 iounmap(default_par->riva.PRAMIN);

Fixes: 1162b070 ("ARC: I/O and DMA Mappings")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Vineet Gupta <vgupta@kernel.org>
Cc: linux-snps-arc@lists.infradead.org
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>