ARM: xen: unexport HYPERVISOR_platform_op function

HYPERVISOR_platform_op() is an inline function and should not
be exported. Since commit 15bfc234 ("modpost: check for
static EXPORT_SYMBOL* functions"), this causes a warning:

WARNING: "HYPERVISOR_platform_op" [vmlinux] is a static EXPORT_SYMBOL_GPL

Instead, export the underlying function called by the static inline:
HYPERVISOR_platform_op_raw.

Fixes: 15bfc234

 ("modpost: check for static EXPORT_SYMBOL* functions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>