irqchip/irq-mvebu-gicp: Allocate enough memory for spi_bitmap

BITS_TO_LONGS() gives us the number of longs we need, but we want to
allocate the number of bytes.

Fixes: a68a63cb ("irqchip/irq-mvebu-gicp: Add new driver for Marvell GICP")
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>