tty: New RISC-V SBI console driver

The RISC-V ISA defines a simple console that is availiable via SBI calls
on all systems.  The SBI console is designed to be availiable at all
times, so while it's most natural to use this as an early printk target
it's also possible to use this as the system console when there isn't a
better one availiable.

This patch adds support for the RISC-V SBI console via the HVC
infastructure.  It's entirely independent from our early printk support,
which results in early boot messages appearing twice over the SBI
console.  As far as I can tell that's the fault of our early printk
support (we should support earlycon) as opposed to this driver.

There is one checkpatch.pl warning here: to check the MAINTAINERS file.
They're all matched by the "K: riscv" line.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>