ARM: topology: Make it clear that all CPUs need to be described

The ARMv8 code will reject incomplete topologies that omit some CPUs (and it's
not clear that it's ever sensible to do so). Update the binding document to
make this clear.

Since we're reformatting the text also fix incorrect grammar in the
final "Any other configuration..." section by removing "consider".

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>