clk: Add support for AST2600 SoC

The ast2600 is a new BMC SoC from ASPEED. It contains many more clocks
than the previous iterations, so support is broken out into it's own
driver.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lkml.kernel.org/r/20190825141848.17346-3-joel@jms.id.au
[sboyd@kernel.org: Mark arrays const]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>