serial: vt8500: Cleanup code using devm_ function

Convert the last memory allocation (vt8500_port) to use devm_kzalloc
and remove the fail path cleanup code from vt8500_serial_probe.

Reorder iomem mapping above clk_enable to simplify fail code. The
clock is only enabled if all other resources are available.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>