clk: tango4: clkgen driver for Tango4 platforms

Provide support for Sigma Designs Tango4 clock generator.
NOTE: This driver is incompatible with Tango3 clkgen.

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
[sboyd@codeaurora.org: Add kernel.h include for panic/sprintf]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>