powerpc/prom_init: Move custom isspace() to its own namespace

If by some reason any of the headers will include ctype.h
we will have a name collision. Avoid this by moving isspace()
to the dedicate namespace.

First appearance of the code is in the commit cf68787b
("powerpc/prom_init: Evaluate mem kernel parameter for early allocation").

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[mpe: Reformat prom_isxdigit() now that we allow longer lines]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210510144925.58195-1-andriy.shevchenko@linux.intel.com