drivers tty: add okl4_vtty driver

Adds a driver which provides console and TTY using the OKL4
hypervisor pipe IPC facility.

Change-Id: Iccbb9eda626fd90d3cc6494c490e13d51edfe42a
Signed-off-by: Carl van Schaik <carl@cog.systems>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Ashok Kundurthi <askund@codeaurora.org>