tty: New RISC-V SBI console driver

The RISC-V ISA defines a simple console that is availiable via SBI calls
on all systems.  This patch adds a driver for this console interface
that can act as both a target for early printk and as the system
console.  The core arch code already enables the early printk support
when CONFIG_HVC_RISCV_SBI is defined.

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@dabbelt.com>