arm: update in-source documentation references

The Arm documentation has moved to Documentation/arch/arm; update
references within arch/arm to match.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Alim Akhtar <alim.akhtar@samsung.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-arch@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>