mfd: sec-core: Allow building as module

The main MFD driver for Samsung PMICs (S2MPSXX, S5M876X) used with
Exynos SoCs can be compiled and used as a module.  The dependent clock,
regulator and RTC drivers already can be built as a module.

Building entire set of drivers as modules might require using initial
ramdisk and can make booting process longer (due to probe deferrals).
However adding such option is useful for testing and for multi-platform
configurations.

This also add required module authors to sec-irq.c file based on recent
main contributors.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>