powerpc: update 512x-defconfig

Activate all MPC512x related boards. Also enable GPIO-driver, SPI driver
and at25 to test SPI. Enable DEVTMPFS. Bump to 3.1-rc6.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>