[POWERPC] Don't store a command line in the Holly device tree

Currently, the Holly device tree includes a bootargs property in
/chosen, which gives a commandline.  This is somewhat inconvenient,
because it means an alternative default command line can't be given in
the kernel config - the value obtained from the dts via the
bootwrapper will always override CONFIG_CMDLINE.

This removes the command line from the dts, and instead puts the
same command line as a default in holly_defconfig.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>