tty/serial: add arm/arm64 semihosting earlycon

Add earlycon support for the arm/arm64 semihosting debug serial
interface. This allows enabling a debug console when early_params are
processed. This is based on the arm64 earlyprintk smh support and is
intended to replace it.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc Documentation/kernel-parameters.txt
Simple merge

diff --cc drivers/tty/serial/Kconfig
Simple merge

diff --cc drivers/tty/serial/Makefile
Simple merge

diff --cc drivers/tty/serial/earlycon-arm-semihost.c
Simple merge