MIPS: ath79: Add missing include file

MIPS: ath79: Add missing include file

Commit ddd0ce87bfde ("mips: Remove unnecessary of_platform_populate with
default match table") dropped the include of linux/clk-provider.h from
arch/mips/ath79/setup.c. This results in the following build error.

arch/mips/ath79/setup.c: In function 'ath79_of_plat_time_init':
arch/mips/ath79/setup.c:232:2: error:
	implicit declaration of function 'of_clk_init'

Fixes: ddd0ce87bfde ("mips: Remove unnecessary of_platform_populate with default match table")
Cc: Rob Herring <robh@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>