clk: davinci: pll: allow dev == NULL

This modifies the TI Davinci PLL clock driver to allow for the case
when dev == NULL. On some (most) SoCs that use this driver, the PLL
clock needs to be registered during early boot because it is used
for clocksource/clkevent and there will be no platform device available.

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/20180525181150.17873-7-david@lechnology.com