serial: vt8500_serial: Convert to devm_ioremap_resource()

Use the newly introduced devm_ioremap_resource() instead of
devm_request_and_ioremap() which provides more consistent error handling.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>