dt/bindings: Correct clk binding example for PIC32 serial.

Update binding example based on new clock binding scheme.
[1] Documentation/devicetree/bindings/clock/microchip,pic32.txt

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Joshua Henderson <digitalpeer@digitalpeer.com>
Cc: Andrei Pistirica <andrei.pistirica@microchip.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13267/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>