Kirkwood: allow to override CONFIG_SYS_TCLK

This patch allow to override CONFIG_SYS_TCLK from board configuration
files. This is needed for the Network Space v2 which use a non standard
core clock frequency (166MHz instead of 200MHz for a 6281 SoC).

As a possible enhancement for 6281 and 6282 devices, TCLK could be
dynamically detected by checking the Sample at Reset register bit 21.

Additionally this patch fix a typo.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Acked-by: Prafulla Wadaskar <Prafulla@marvell.com>