plat-vexpress: configure CFG_CORE_ASYNC_NOTIF_GIC_INTID

When compiled for SPMC at S-EL1 (CFG_CORE_SEL1_SPMC=y), configure
CFG_CORE_ASYNC_NOTIF_GIC_INTID to an unused secure SGI that can be
donated to the normal world.

In boot_primary_init_intc(), only donate the interrupt id if it's in the
predefined secure SGI range.

Fixes: 462028ed

 ("qemu_armv8a: add GIC v3 redistributor base address")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>