efsys.h may depends on target system headers only. Otherwise, it could
result in cross-dependency when libefx header depends on efsys.h and
efsys.h depends on the libefx header.
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com> Reviewed-by: Andrew Lee <andrewle@xilinx.com> Reviewed-by: Andy Moreton <amoreton@xilinx.com>