tty: Update hypervisor tty drivers to use core stdout parsing code.

The evh_bytechan, hvc_opal and hvc_vio drivers all open code the parsing
of the stdout node in the device tree. This patch simplifies the driver
by removing the duplicated functionality.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
(cherry picked from commit a752ee56ad84bf9a35b8323af1ad22b03c1df2c4)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
5 files changed