dt-bindings: reset: Fix brcmstb-reset example

The reset controller has a #reset-cells value of 1, so we should see a
phandle plus a register identifier, fix the example.

Fixes: 0807caf6 ("dt-bindings: reset: Add document for Broadcom STB reset controller")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>