doc: command 'ftdi_layout_signal' can run after config

doc: command 'ftdi_layout_signal' can run after config

The mode of command 'ftdi_layout_signal' is set to COMMAND_ANY in
[1] and the command has no constraints that limits its use to the
config phase only.
But [2] documents it as '{Config Command}'.

Update the documentation to report the correct mode '{Command}'.

[1] commit f5e97b5e1b62 ("Add FTDI JTAG driver using MPSSE layer")
[2] commit 76afadeb7b4e ("doc: Add documentation for the ftdi driver")

Change-Id: I6b14aebb98e48f7080c585f3df881714bb188af0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6155
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>