clk: bcm63xx-gate: switch to dt-bindings definitions

Now that there are header files for each SoC, let's use them in the
bcm63xx-gate controller driver.
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Link: https://lore.kernel.org/r/20200615090231.2932696-9-noltari@gmail.comAcked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>