xen/arm: head: Add missing isb after writing to SCTLR_EL2/HSCTLR

xen/arm: head: Add missing isb after writing to SCTLR_EL2/HSCTLR

Write to SCTLR_EL2/HSCTLR may not be visible until the next context
synchronization. When initializing the CPU, we want the update to take
effect right now. So add an isb afterwards.

Spec references:
    - AArch64: D13.1.2 ARM DDI 0406C.d
    - AArch32 v8: G8.1.2 ARM DDI 0406C.d
    - AArch32 v7: B5.6.3 ARM DDI 0406C.d

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>