ramips: speed up spi frequency for Youku YK-L1

ramips: speed up spi frequency for Youku YK-L1

Youku YK-L1 has a huge storage space up to 32 MB. It is better to
use a higher spi clock to read or write serial nor flash chips.
Youku YK-L1 has Winbond w25q256fvfg on board that can support
104 MHz spi clock so 48 MHz is safe enough.
The real frequency can only be sysclk(580MHz ) /3 /(2^n) so 80 MHz
defined in dts file will set only 48 MHz in spi bus.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>