Move the PS7 inititalization into its own function. This helps
readability and logically splits the FPGA toolchain generated
setup from the reset of the board init.

Also execute the PS7 setup after the lowlevel CPU init, as this
is the regular order used in the Barebox codebase.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>