ARM: OMAP: Add initialisation for the real-time counter.

The real time counter also called master counter, is a free-running
counter. It produces the count used by the CPU local timer peripherals
in the MPU cluster. The timer counts at a rate of 6.144 MHz.

The ratio registers needs to be configured based on system clock
only onetime. After initialisation, hardware takes care of adjusting
the clock in different low power modes to keep counter rate constant.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>