arm: omap-common: do not build timer when CONFIG_TIMER defined

To prepare timer driver to DM/DT conversion do not build the
exiting timer driver when CONFIG_TIMER is defined. But since
omap's SPL doesn't support DM yet so built timer driver only for
SPL build when CONFIG_TIMER is defined.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>