drivers: fpga: zynqpl: fix compilation with SPL

Disable the use of function zynq_loadfs when compiling
the driver for the SPL, as the following filesystem
functions are not found by the linker:
- fs_set_blk_dev
- fs_read
- fs_set_blk_dev
- fs_read
- fs_read

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>