clk: ti: am33xx: fix RTC clock parent

clk: ti: am33xx: fix RTC clock parent

Right now, trying to use RTC purely with the ti-sysc / clkctrl framework
fails to enable the RTC module properly. Based on experimentation, this
appears to be because RTC is sourced from the clkdiv32k optional clock.
TRM is not very clear on this topic, but fix the RTC to use the proper
source clock nevertheless.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Link: https://lkml.kernel.org/r/20200424152301.4018-1-t-kristo@ti.com
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>