arm: kirkwood: Do not overwrite CONFIG_SYS_TCLK
    
    Config option CONFIG_SYS_TCLK is set by kw88f6281.h and kw88f6192.h files
    to correct SOC/platform value. So do not overwrite it in board config
    include files.
    
    Kirkwood 88F6180 and 88F6192 uses 166 MHz TCLK and Kirkwood 88F6281 uses
    200 MHz TCLK.
    
    Signed-off-by: Pali Rohár <pali@kernel.org>
    Reviewed-by: Stefan Roese <sr@denx.de>