powerpc: Disallow lparcfg being a module

The lparcfg code needs several things which are pretty arcane internal
details and which we don't want to export, which means that lparcfg
doesn't work when built as a module.  This makes it a bool instead of
a tristate in the Kconfig so that users can't try to build it as a
module.

Signed-off-by: Paul Mackerras <paulus@samba.org>