arm{,64}/xen: Remove "EXPERIMENTAL" in the description of the Xen options

The Xen ARM API is stable since Xen 4.4 and everything has been
upstreamed in Linux for ARM and ARM64. Therefore we can drop "EXPERIMENTAL"
from the Xen option in the both Kconfig.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org