serial: amba-pl010: Use devres APIs

Migrating to use devres managed APIs devm_kzalloc, devm_ioremap and
devm_clk_get.

Signed-off-by: Tushar Behera <tushar.b@samsung.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>