[POWERPC] maple: Use platform name in define_machine()

Prevailing practice for define_machine() in powerpc is to use the
platform name when the platform has only one define_machine()
statement, but maple uses "maple_md".  This caused me some
head-scratching when writing some new code that uses
machine_is(maple).

Use "maple" instead of "maple_md".  There should not be any behavioral
change -- fixup_maple_ide() calls machine_is(maple) but the body of
the function is ifdef'd out.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>