clk: qoriq: provide constants for the type

To avoid future mistakes in the device tree for the clockgen module, add
constants for the clockgen subtype as well as a macro for the PLL
divider.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201108185113.31377-4-michael@walle.cc
Signed-off-by: Stephen Boyd <sboyd@kernel.org>