ARM: Xilinx: merge board file into main platform code

The zynq platform will never have board files other than the
device tree one, so there is no point splitting it from common.c.

This makes the code more compact.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: John Linn <john.linn@xilinx.com>