ARM: zynq: Remove sparse warnings

ARM: zynq: Remove sparse warnings

Warnings:
board/xilinx/zynq/board.c:17:13: warning: symbol 'fpga' was not declared. Should it be static?
board/xilinx/zynq/board.c:20:13: warning: symbol 'fpga010' was not declared. Should it be static?
board/xilinx/zynq/board.c:21:13: warning: symbol 'fpga015' was not declared. Should it be static?
board/xilinx/zynq/board.c:22:13: warning: symbol 'fpga020' was not declared. Should it be static?
board/xilinx/zynq/board.c:23:13: warning: symbol 'fpga030' was not declared. Should it be static?
board/xilinx/zynq/board.c:24:13: warning: symbol 'fpga045' was not declared. Should it be static?
board/xilinx/zynq/board.c:25:13: warning: symbol 'fpga100' was not declared. Should it be static?
board/xilinx/zynq/board.c:120:5: warning: symbol 'board_mmc_init' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>