mfd: sec: Initialize driver via module_platform_driver()

The driver was using subsys_initcall() because in old times deferred
probe was not supported everywhere and specific ordering was needed.
Since probe deferral works fine and specific ordering is discouraged
(hides dependencies between drivers and couples their boot order), the
driver can be converted to regular module_platform_driver.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>