clk: davinci: psc: allow for dev == NULL

On some davinci SoCs, we need to register the PSC clocks during early
boot because they are needed for clocksource/clockevent. These changes
allow for dev == NULL because in this case, we won't have a platform
device for the clocks.

Signed-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20180525181150.17873-9-david@lechnology.com