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: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>