spi: aspeed: Clock frequency adjustment support

Driver can configure the SPI clock frequnecy to the
target value of "spi-max-frequency" property in
the device tree. The frequency is divided from HCLK,
200MHz. Usually, the ASPEED SPI clock frequency range
is between 12.5MHz and 100MHz. On AST2600, the lowest
SPI clock frequency can be about 780kHz.

Signed-off-by: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>