clk: pistachio: Make it selectable for generic MIPS kernel

We're moving pistachio to generic MIPS kernel. The clk driver
should be avilable to the generic MIPS kernel.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>