clk: bcm281xx: move compatible string definitions

The Broadcom 281xx clock code uses a #define for the compatible
string for it's clock control units (CCUs).  Rather than defining
those in the C source file, define them in the header file that's
shared by both the code and the device tree source file (along with
all the clock ids).

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>