dt/flattree: explicitly pass command line pointer to early_init_dt_scan_chosen

This patch drops the reference to a global 'cmd_line' variable from
early_init_dt_scan_chosen, and instead passes the pointer to the command
line string via the *data argument.  Each architecture does something
slightly different with the initial command line, so it makes sense for
the architecture to be able to specify the variable name.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>