Xilinx: ARM: devcfg: Added support for partial bitstreams

This patch creates a syfs entry is_partial_bitstream to
indicate whether bitstream is partial or not. Based on the
value of is_partial_bitstream, PROG_B is asserted and
slcr_init_preload_fpga and slcr_init_postload_fpga functions
are invoked.

Signed-off-by: Suneel <suneelg@xilinx.com>