arm925t: Fix CONFIG_SYS_HZ to 1000

Let CONFIG_SYS_HZ to have value of 1000 effectively fixing all users of
get_timer.

Changes since original version:
* Set PTV=2 (divisor 8) for boards using 12MHz timer clock source to
  improve timer resolution.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>