dt-bindings: clk: Add Baikal-T1 CCU PLLs binding

dt-bindings: clk: Add Baikal-T1 CCU PLLs binding

Baikal-T1 Clocks Control Unit is responsible for transformation of a
signal coming from an external oscillator into clocks of various
frequencies to propagate them then to the corresponding clocks
consumers (either individual IP-blocks or clock domains). In order
to create a set of high-frequency clocks the external signal is
firstly handled by the embedded into CCU PLLs. So the corresponding
dts-node is just a normal clock-provider node with standard set of
properties. Note as being part of the Baikal-T1 System Controller its
DT node is supposed to be a child the system controller node.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-mips@vger.kernel.org
Link: https://lore.kernel.org/r/20200526222056.18072-2-Sergey.Semin@baikalelectronics.ru
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>