clk: Provide struct clk for fixed rate clock (clk_fixed_rate.c)

Up till now the fixed rate clock ('osc') has been added to UCLASS_CLK
without declaring struct clk. As a result it was only accessible by
iterating the udevice's uclass list.

This is a problem for clock code, which operates on pointers to struct
clk (like clk_get_rate()), not udevices.

After this change struct clk is accessible from udevice and udevice from
struct clk.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>