u-boot-aspeed: Add distro feature to enable SPI master mode

The ast2500 shares the RGMII1 pin and the hw strap pins
for SPI interface mode selection ( pin[12:13] ).
In some systems, the RGMII/NCSI interface will use the pin.
It makes the SPI interface mode setting is not correct.

This patch adds a distro feature to enable the SPI master
mode by default.

Signed-off-by: Chanh Nguyen <chanh@os.amperecomputing.com>
Change-Id: I93e5dd5e86870601169974aa1aab4b5480a45ef1