ARM: dts: am43xx-clocks: Fix ehrpwm tbclk data on am43xx

ehrpwm tbclk is wrongly modelled as deriving from dpll_per_m2_ck.
The TRM says tbclk is derived from SYSCLKOUT. SYSCLKOUT nothing but the
functional clock of pwmss (l4ls_gclk).
Fix this by changing source of ehrpwmx_tbclk to l4ls_gclk.

Fixes: 4da1c677 ("add tbclk data for ehrpwm")
Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>