Documentation: arm64: Add limitation for exactmap

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I56F4Z


CVE: NA

--------------------------------

For arm64, the exactmap has to remove all memblock and then add
the user assigned range of memblock. EFI initializes memblock too
late so removing all memblock after it will cause panic. Hence,
the exactmap can be just used in dt boot mode, add this limitation
to Documentation/admin-guide/kernel-parameters.txt.

Signed-off-by: Peng Liu <liupeng256@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>