ARM: OMAP2+: DRA7: realtime_counter: Add ratio registers for 20MHZ sys-clk frequency

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 are missing for a sys-clk of 20MHZ which is used
by DRA7 socs. So because of this, the counter was getting wrongly
programmed for a sys-clk of 38.4Mhz(default). So adding the ratio
registers for 20MHZ sys-clk.
Tested-by: Nishanth Menon <nm@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/timer.c

@@ -515,6 +515,10 @@ static void __init realtime_counter_init(void)
 		num = 8;
 		den = 25;
 		break;
+	case 20000000:
+		num = 192;
+		den = 625;
+		break;
 	case 2600000:
 		num = 384;
 		den = 1625;