MIPS: Alchemy: Include clk.h

This clock provider uses the consumer API, so include clk.h
explicitly.

Cc: Manuel Lauss <manuel.lauss@gmail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>