ARM: OMAP2+: Simplify system timer clock definitions

In commit c59b537d (ARM: OMAP2+: Simplify dmtimer clock aliases), new
clock aliases for dmtimers were added to simplify the code. These clock
aliases can also be used when configuring the system timers and allow us
to remove the current definitions, simplifying the code.

Please note that for OMAP4/5 devices (unlike OMAP2/3 devices), there is
no clock alias for "timer_sys_ck" with NULL as the device name. Therefore
we still need to use the alias "sys_clkin_ck" for these devices.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>