clk: davinci: Fix link errors when not all SoCs are enabled

This fixes linker errors due to undefined symbols when one or more of
the TI DaVinci SoCs is not enabled in the kernel config.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20180525181150.17873-10-david@lechnology.com