serial: ns16550: Add RX interrupt buffer support

Pasting longer lines into the U-Boot console prompt sometimes leads to
characters missing. One problem here is the small 16-byte FIFO of the
legacy NS16550 UART, e.g. on x86 platforms.

This patch now introduces a Kconfig option to enable RX interrupt
buffer support for NS16550 style UARTs. With this option enabled, I was
able paste really long lines into the U-Boot console, without any
characters missing.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
[trini: Guard ns16550_serial_remove with
CONFIG_IS_ENABLED(SERIAL_PRESENT) to match struct assignment]
Signed-off-by: Tom Rini <trini@konsulko.com>