haven: add sysfs and debug interfaces

This patch adds the /sys/hypervisor sysfs interface for the Haven
hypervisor. When running hosted on the Haven hypervisor, this interface
provides the Haven API and version information, and an interface for
controlling debug features. Haven also provides a call to disable its
UART driver. Disabling Haven's UART driver is exposed through a Kconfig
option.

Change-Id: I2a5ca7b01487e01e2d1dd56f5d15e826e70af081
Signed-off-by: Elliot Berman <eberman@codeaurora.org>