ARM: exynos: dts: Update 5250 arch timer node with clock frequency

Without the "clock-frequency" property in arch timer node, could able
to see the below crash dump.

[<c0014e28>] (unwind_backtrace+0x0/0xf4) from [<c0011808>] (show_stack+0x10/0x14)
[<c0011808>] (show_stack+0x10/0x14) from [<c036ac1c>] (dump_stack+0x7c/0xb0)
[<c036ac1c>] (dump_stack+0x7c/0xb0) from [<c01ab760>] (Ldiv0_64+0x8/0x18)
[<c01ab760>] (Ldiv0_64+0x8/0x18) from [<c0062f60>] (clockevents_config.part.2+0x1c/0x74)
[<c0062f60>] (clockevents_config.part.2+0x1c/0x74) from [<c0062fd8>] (clockevents_config_and_register+0x20/0x2c)
[<c0062fd8>] (clockevents_config_and_register+0x20/0x2c) from [<c02b8e8c>] (arch_timer_setup+0xa8/0x134)
[<c02b8e8c>] (arch_timer_setup+0xa8/0x134) from [<c04b47b4>] (arch_timer_init+0x1f4/0x24c)
[<c04b47b4>] (arch_timer_init+0x1f4/0x24c) from [<c04b40d8>] (clocksource_of_init+0x34/0x58)
[<c04b40d8>] (clocksource_of_init+0x34/0x58) from [<c049ed8c>] (time_init+0x20/0x2c)
[<c049ed8c>] (time_init+0x20/0x2c) from [<c049b95c>] (start_kernel+0x1e0/0x39c)

THis is because the Exynos u-boot, for example on the Chromebooks, doesn't set
up the CNTFRQ register as expected by arch_timer. Instead, we have to specify
the frequency in the device tree like this.

Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@samsung.com>
[olof: Changed subject, added comment, elaborated on commit message]
Signed-off-by: Olof Johansson <olof@lixom.net>