cmd: fpga: Add support for missing fpga loadmk commands

There are ways how to call fpga loadmk

1. Full command
fpga loadmk [dev] [address]

2. Dev setup via variable
set fpga [dev]
fpga loadmk [address]

3. Address setup via variable
set fpgadata [address]
fpga loadmk [dev]

4. Dev and address setup via variables
set fpga [dev]
set fpgadata [address]
fpga loadmk

Before this patch only cases 1 and 3 are working but the part of code
was trying to support also cases 2 and 4.
This patch is adding support for cases 2 and 4 to have all of
combinations supported.

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