rtc: rtc-vt8500: use devm_ioremap_resource()

Use devm_ioremap_resource() in order to make the code simpler, and move
'struct resource *res' from 'struct vt8500_rtc' to vt8500_rtc_probe()
because the 'res' variable is used only in vt8500_rtc_probe().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>