omap1: Fix booting for 15xx and 730 with omap1_defconfig

For omap15xx and 730 we need to use the MPU timer
as the 32K timer is not available. For omap16xx
we want to use the 32K timer because of PM. Fix this
by allowing to build in both timers.

Signed-off-by: Tony Lindgren <tony@atomide.com>