irqchip/irq-mvebu-sei: Make mvebu_sei_ap806_caps static

Fix sparse warning:

drivers/irqchip/irq-mvebu-sei.c:481:23: warning:
 symbol 'mvebu_sei_ap806_caps' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <jason@lakedaemon.net>
Cc: <andrew@lunn.ch>
Cc: <gregory.clement@bootlin.com>
Cc: <sebastian.hesselbarth@gmail.com>
Cc: <marc.zyngier@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Link: https://lkml.kernel.org/r/20190321151448.15600-1-yuehaibing@huawei.com