clk: ti: consider the fact that of_clk_get() might return an error

I "forgot" to update the dtb and the kernel crashed:
|Unable to handle kernel NULL pointer dereference at virtual address 0000002e
|PC is at __clk_get_flags+0x4/0xc
|LR is at ti_dt_clockdomains_setup+0x70/0xe8

because I did not have the clock nodes. of_clk_get() returns an error
pointer which is not checked here.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tero Kristo <t-kristo@ti.com>