omap-common/hwinit-common.c: timer_init() doesn't need to touch gd

The gd will be cleared at first so we don't need to set arch.tbl to 0.
In addition, the checks later against lastinc also work fine with an
initial value of 0 here.  This also brings us in line with sunxi code
for example.

Signed-off-by: Tom Rini <trini@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>