serial/arc: Refactor by referencing to uart_port where possible

The ARC UART MMIO helpers would take arc_uart_port and then reference
generic uart_port->membase member. So change them to difrectly refer to
uart_port and fix call sites accordingly.

This removes the need for to_arc_port() converion almost eveeywhere and
makes code a bit easier to read.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>