Prior to this commit, ZC702 and ZC706 shared the same configuration
and were built as follows:

ZC702: make zynq_zc70x_defconfig && make
ZC706: make zynq_zc70x_defconfig && make DEVICE_TREE=zynq-zc706

This commit introduces separate configuration for them, which makes
the next commit much easier.

Going forward, the recommended build commands are:

ZC702: make zynq_zc702_defconfig && make
ZC706: make zynq_zc706_defconfig && make

Although the old work flow is still supported, CONFIG_TARGET_ZC70X
has been marked as deprecated.  If used, the warning message is
shown to prompt users to switch to the new scheme.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>