platforms/xgene: Make use of dt_device_get_address return value

platforms/xgene: Make use of dt_device_get_address return value

Currently function xgene_check_pirq_eoi assigns the return value of
dt_device_get_address to a variable res but does not make use of it.
Fix it by making use of res in the condition checking the result of a
call to dt_device_get_address instead of checking the address stored in
dbase.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>