Revert "arch: arm64: init xen in arch_kernel_init()"

This reverts commit 7c90f1bc.

Xen initialization maps enlighten page to Zephyr memory and also
initializes Xen event channels. It is used for communication between
Xen domains and based on interrupt connected to domain virtual GIC.
Moving event channel initialization to arch_kernel_init() make it call
irq_enable() when GIC is not initialized. Since GIC is initialized
on PRE_KERNEL_1 stage, this lead to fatal error during boot.

Revert these changes to make xenvm operable again.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>