clk: ti: DRA7: Add timer_sys_ck aliases for Timers 13 through 16

The OMAP DMTimer API, omap_dm_timer_set_source(), uses the clock name
timer_sys_ck for setting a timer's clock source for the source index
OMAP_TIMER_SRC_SYS_CLK. There is currently no clock alias data for
the Timers 13 through 16 for this clock name, so add the same.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>