serial: serial-uclass: Add generic serial RX buffer support

serial: serial-uclass: Add generic serial RX 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 buffer support
for all DM based serial drivers. 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 <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Tested-by: Bin Meng <[email protected]>