arm: cast int<->pointer via uintptr_t

arm: cast int<->pointer via uintptr_t

Fixes build on arm64:

gic.c: In function ‘gic_irq_eoi’:
        gic.c:726:16: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
        gic.c: In function ‘maintenance_interrupt’:
        gic.c:776:29: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
        gic.c:778:64: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
        cc1: all warnings being treated as errors
        make[3]: *** [gic.o] Error 1

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>