mfd: sec: Remove unused irq_base in platform data

The 'irq_base' field of platform data structure is not assigned,
therefore its default value of 0 has no impact and can be safely
dropped.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>