Documentation: arm: Replace use of virt_to_phys with __pa_symbol

All low-level PM/SMP code using virt_to_phys() should actually use
__pa_symbol() against kernel symbols.  Update the documentation to move
away from virt_to_phys().

Cfr. commit 6996cbb2372189f7 ("ARM: 8641/1: treewide: Replace uses of
virt_to_phys with __pa_symbol")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>