misc: vexpress/syscfg: Use devm_ioremap_resource() to map memory

Instead of checking the return value of platform_get_resource(), we can
use devm_ioremap_resource() which has the NULL pointer check and the
memory region requesting. devm_ioremap_resource is designed to replace
calls to devm_request_mem_region followed by devm_ioremap, so let's use
the same.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>