arm: interrupt_init: set sp in IRQ/FIQ modes

Before this commit, the stack addresses for IRQ and FIQ modes,
IRQ_STACK_START and FIQ_STACK_START, were computed in interrupt_init but
they were not used.

This commit sets the stack pointers for IRQ and FIQ modes.

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>