dts: bindings: trivial clock bindings doc fixes

fix a typo in the "clock specifiers" discussion,
clarify that clock specifiers (the integer cells
part that goes with the phandle) may be empty

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: devicetree@vger.kernel.org
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Rob Herring <robh@kernel.org>