arm: mvebu: a37x: Detect CONFIG_SYS_TCLK from SAR register
    
    Bit 20 in SAR register specifies if TCLK is running at 200 MHz or 166 MHz.
    Use this information instead of manual configuration in every board file.
    
    Signed-off-by: Pali Rohár <pali@kernel.org>
    Reviewed-by: Stefan Roese <sr@denx.de>