microblaze: Fix early cmdline for CMDLINE_FORCE

This patch fixed parsing early parameters because
current implementation does that early parse DTS
command line and then parse CMDLINE line which is compiled-in.

For case that DTS doesn't contain command line is
copied command line from kernel with is done in prom.c
that's why I can remove it from machine_early_init.

Signed-off-by: Michal Simek <monstr@monstr.eu>