mfd: cpcap: Fix compile if MFD_CORE is not selected

If only cpcap mfd driver is selected we will get:

ERROR: "devm_mfd_add_devices" [drivers/mfd/motorola-cpcap.ko] undefined!

This is because Kconfig is missing select for MFD_CORE.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>