clk: Add Baikal-T1 CCU Dividers driver

clk: Add Baikal-T1 CCU Dividers driver

Nearly each Baikal-T1 IP-core is supposed to have a clock source
of particular frequency. But since there are greater than five
IP-blocks embedded into the SoC, the CCU PLLs can't fulfill all the
needs. Baikal-T1 CCU provides a set of fixed and configurable clock
dividers in order to generate a necessary signal for each chip
sub-block.

This driver creates the of-based hardware clocks for each divider
available in Baikal-T1 CCU. The same way as for PLLs we split the
functionality up into the clocks operations (gate, ungate, set rate,
etc) and hardware clocks declaration/registration procedures.

In accordance with the CCU documentation all its dividers are distributed
into two CCU sub-blocks: AXI-bus and system devices reference clocks.
The former sub-block is used to supply the clocks for AXI-bus interfaces
(AXI clock domains) and the later one provides the SoC IP-cores reference
clocks. Each sub-block is represented by a dedicated DT node, so they
have different compatible strings to distinguish one from another.

For some reason CCU provides the dividers of different types. Some
dividers can be gateable some can't, some are fixed while the others
are variable, some have special divider' limitations, some've got a
non-standard register layout and so on. In order to cover all of these
cases the hardware clocks driver is designed with an info-descriptor
pattern. So there are special static descriptors declared for the
dividers of each type with additional flags describing the block
peculiarity. These descriptors are then used to create hardware clocks
with proper operations.

Some CCU dividers provide a way to reset a domain they generate
a clock for. So the CCU AXI-bus and CCU system devices clock
drivers also perform the reset controller registration.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20200526222056.18072-5-Sergey.Semin@baikalelectronics.ru
[sboyd@kernel.org: Drop return from void function, silence sparse
warnings about initializing structs with NULL vs. integer]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>