clk: iproc: Extend binding to cover BCM63138

Broadcom BCM63138 DSL SoCs have the same ARMPLL clocking infrastructure
as the Cygnus and iProc chips, add a dedicated compatible string and
document that the ARMPLL node is a valid node for this chip.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>