dwc3-generic: Don't fail probe if clk/reset entries are absent

Some boards don't populate clk/reset entries as these are are optional
as per binding documentation. Therefore, don't fail driver probe if
clk/reset entries are absent in DT.

This fixes fastboot failures seen due to enabling of CONFIG_CLK on AM57xx

Fixes: e8e683d3 ("board: ti: am57xx-idk: Configure the CDCE913 clock synthesizer")
Reported-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>