arm64: armv8_deprecated: update the comments of armv8_deprecated_init()

In commit c0d8832e

 ("arm64: Ensure the instruction emulation is
ready for userspace"), armv8_deprecated_init() was promoted to
core_initcall() but the comments were left unchanged, update it now.

Spotted by some random reading of the code.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
[will: "can guarantee" => "guarantees"]
Signed-off-by: Will Deacon <will@kernel.org>