xen/arm: Add support of PSCI v1.0 for the host

xen/arm: Add support of PSCI v1.0 for the host

From Xen's point of view, PSCI v0.2 and PSCI v1.0 are very similar. All
the PSCI calls used within Xen (PSCI_VERSION, CPU_ON, SYSTEM_OFF and
SYSTEM_RESET) behave exactly the same.

Furthermore, based on the spec (5.3.1 DEN0022C), any 1.y version must be
compatible with 1.x when y > x for any functions existing in 1.x.

So check the presence of the new compatible string [1] and allow Xen to
boot on any platform using PSCI 1.x.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-October/374547.html

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>