xen: introduce xen,enhanced dom0less property

xen: introduce xen,enhanced dom0less property

Introduce a new "xen,enhanced" dom0less property to enable/disable PV
driver interfaces for dom0less guests. Currently only "enabled" and
"disabled" are supported property values (and empty). Leave the option
open to implement further possible values in the future (e.g.
"xenstore" to enable only xenstore.) Dom0 presence is required for now
to use "xen,enhanced" on a domU.

The configurable option is for domUs only. For dom0 we always set the
corresponding property in the Xen code to true (PV interfaces enabled.)

This patch only parses the property. Next patches will make use of it.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>
CC: Julien Grall <julien@xen.org>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
CC: Bertrand Marquis <bertrand.marquis@arm.com>