cmd: fpga: Extract fpga info command to separate function

cmd: fpga: Extract fpga info command to separate function

Move fpga info to U_BOOT_CMD_MKENT subcommand.
Also use strtol instead of simple_strtoul. The reason is that if -1 is
passed (or fpga info without "fpga" variable) the list of all fpgas is
shown.
This functionality is in the fpga core but it couldn't be performed.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>