mfd: ab8500-core should depend on MFD_DB8500_PRCMU

A recent move to eliminate excess historical baggage from ab8500 core
code resulting in errors when building with x86_64 allmodconfig:

In file included from drivers/mfd/ab8500-core.c:21:0:
include/linux/mfd/dbx500-prcmu.h:614:19: error: redefinition of 'prcmu_abb_read'
include/linux/mfd/db8500-prcmu.h:673:19: note: previous definition of 'prcmu_abb_read' was here
include/linux/mfd/dbx500-prcmu.h:619:19: error: redefinition of 'prcmu_abb_write'
include/linux/mfd/db8500-prcmu.h:678:19: note: previous definition of 'prcmu_abb_write' was here
include/linux/mfd/dbx500-prcmu.h:630:19: error: redefinition of 'prcmu_config_clkout'
include/linux/mfd/db8500-prcmu.h:643:19: note: previous definition of 'prcmu_config_clkout' was here
include/linux/mfd/dbx500-prcmu.h:692:20: error: redefinition of 'prcmu_ac_wake_req'
include/linux/mfd/db8500-prcmu.h:683:20: note: previous definition of 'prcmu_ac_wake_req' was here
include/linux/mfd/dbx500-prcmu.h:694:20: error: redefinition of 'prcmu_ac_sleep_req'
include/linux/mfd/db8500-prcmu.h:685:20: note: previous definition of 'prcmu_ac_sleep_req' was here

Problem:
When CONFIG_AB8500_CORE is set, building ab8500-core.c and
!(CONFIG_UX500_SOC_DB8500 | CONFIG_MFD_DB8500_PRCMU), both db8500-prcmu.h
and dbx500-prcmu.h take it upon themselves to _both_ create 'return 0'
inline functions for the following:

prcmu_abb_read()
prcmu_abb_write()
prcmu_config_clkout()
prcmu_ac_wake_req()
prcmu_ac_sleep_req()

Solution:
Depend on MFD_DB8500_PRCMU, which in turn depends on UX500_SOC_DB8500.

Reported-By: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>