powerpc/mm/radix: Update command line parsing for disable_radix

kernel parameter disable_radix takes different options
disable_radix=yes|no|1|0  or just disable_radix.

prom_init parsing is not supporting these options.

Fixes: 1fd6c022 ("powerpc/mm: Add a CONFIG option to choose if radix is used by default")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>