MIPS: BCM63xx: CLK: Add dummy clk_{set,round}_rate() functions

Several drivers use the clk_{set,round}_rate() functions
that need to be defined in the platform's clock code.
The Broadcom BCM63xx platform hardcodes the clock rate so
we create new clk_{set,round}_rate() functions
which just return 0 like those in include/linux/clk.h
for the common clock framework do.

Also fixes the following build problem on a randconfig:
drivers/built-in.o: In function `nop_usb_xceiv_probe':
phy-nop.c:(.text+0x3ec26c): undefined reference to `clk_set_rate'

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5580/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>