ARM: meson: serial: convert iounmap to devm_iounmap

The function meson_uart_release_port() inappropriately try to
iounmap() a resource managed by devm_ioremap_nocache().
The function meson_uart_release_port() maybe called by uart_ioctl()
that means meson_uart_release_port() is not called from within a
probe or remove function, for safety, I convert iounmap() to
devm_iounmap().

Signed-off-by: Firo Yang <firogm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>