[CPUFREQ] sc520_freq.c: ioremap balanced with iounmap

ioremap must be balanced by an iounmap and failing to do so can result
in a memory leak.

Tested (compilation only):
- using allmodconfig
- making sure the files are compiling without any warning/error due to
new changes

Signed-off-by: Amol Lad <amol@verismonetworks.com>
Signed-off-by: Dave Jones <davej@redhat.com>