OMAP SPL: Fix missing timer_init() call in OMAP4 s_init()

In 8775471b, the call to timer_init() was removed from common code
and put in OMAP3 s_init() function. As a result the boot was broken
on OMAP4. This patch adds timer_init() in OMAP4 s_init(), that fix
boot on all OMAP4 boards.

Signed-off-by: Nicolas Dechesne <n-dechesne@ti.com>
Tested-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Cc: Tom Rini <trini@ti.com>