ARM: OMAP3: Add generic machine descriptor for boards with OMAP3 GP devices

OMAP3 devices may or may not have security features enabled. Security enabled
devices are known as high-secure (HS) and devices without security are known as
general purpose (GP).

Some OMAP3 boards, such as the OMAP3 beagle board, only use GP devices and for
GP devices there is a 12th timer available on-chip that can operate at 32kHz.
The clock for 12th timer is generated by an internal oscillator and is unique
this timer. Boards such as the beagle board use this timer as a 32kHz based
clock-events timer because early versions of the board had a hardware problem
preventing them from using other on-chip timers clocked by a external 32kHz
clock.

When booting with device-tree all OMAP3 devices use timer 1 by default for
the clock-events timer. Therefore, add a generic machine descriptor for boards
with OMAP3 GP devices so that they can use the 12th timer as the clock-events
timer instead of the default.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>