mfd: sec: Move private internal API to internal header

mfd: sec: Move private internal API to internal header

sec_irq_init() is an internal API for the core driver, and doesn't
belong into the public header.

Due to an upcoming split of the driver into a core and i2c driver,
we'll also be adding more internal APIs, which again shouldn't be in
the public header.

Move it into a new internal include.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-7-d66d5f39b6bf@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>