It makes sense to specify CONFIG_SYS_CLK_FREQ in "configs/xx_defconfig"
instead of "include/configs/xxx.h" because then header will be reusable
across boards with different CPU clocks.

Also this nice to have an ability for end user to tune this value
himself via "menuconfig".

For now I'm only applying this change to all ARC configs because
otherwise scope of change will be huge.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Tom Rini <trini@ti.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Hans de Goede <hdegoede@redhat.com>
cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>