powercap / RAPL: fix build dependency on iosf_mbi

RAPL on Atom depends on IOSF_MBI driver for setting
floor frequency. This patch adds explicit dependency on
CONFIG_IOSF_MB.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>