ARM: zynq: pass "-mfpu=neon" only to lowlevel_init.S

The comment line in arch/arm/cpu/armv7/zynq/config.mk says that
the option "-mfpu=neon" is necessary for compiling lowlevel_init.S.
We do not have to give it to all the source files.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>