[POWERPC] Minor fix for bootargs property

Avoid the use of an uninitialized stack variable when the powerpc device tree
bootargs property is either missing or incorrectly defined.  This also makes
CONFIG_CMDLINE work properly under these conditions.  This change adds a test
for the existence of the bootargs property.

early_init_dt_scan_chosen() tests for a zero length bootargs property in its
CONFIG_CMDLINE processing, but the current implementation of
of_get_flat_dt_prop() doesn't assign a value to the length when no property is
found.  Since an automatic variable is used, a stale value from the stack will
be used in the test.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>