serial: liteuart: Add support for earlycon

Most litex boards using RISC-V soft cores us the sbi earlycon, however
this is not available for non RISC-V litex SoC's.  This patch enables
earlycon for liteuart which is available on all Litex SoC's making
support for earycon debugging more widely available.

Cc: Florent Kermarrec <florent@enjoy-digital.fr>
Cc: Mateusz Holenko <mholenko@antmicro.com>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Gabriel L. Somlo <gsomlo@gmail.com>
Reviewed-and-tested-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stafford Horne <shorne@gmail.com>
Link: https://lore.kernel.org/r/20210517115453.24365-1-shorne@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>