Documentation: arm64: Describe the support and expectations for PBHA

maillist inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7ZC0H

Reference: https://lore.kernel.org/all/20211015161416.2196-1-james.morse@arm.com/t/#u



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PBHA isn't defined by the Arm CPU architecture, so may have surprising
side-effects.

Document what is, and what is not supported. List the arch code's
expectations regarding how PBHA behaves.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>