arm64: mm: Export __sync_icache_dcache() for xen-privcmd

The xen-privcmd driver, which can be modular, calls set_pte_at()
which in turn may call __sync_icache_dcache().

The call to __sync_icache_dcache() may be optimised out because it is
conditional on !pte_special(), and xen-privcmd calls pte_mkspecial().
But it seems unwise to rely on this optimisation.

Fixes: 3ad08765

 ("xen/privcmd: add IOCTL_PRIVCMD_MMAP_RESOURCE")
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>