mfd: Fix 88PM8607 I2C dependency

88PM8607 depends on I2C and MFD_CORE. Since 88PM8607 is built-in
kernel, it also requires that I2C and MFD_CORE are built-in
kernel also.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>