powerpc: export ppc_proc_freq and ppc_tb_freq as GPL symbols

Export the global variable 'ppc_tb_freq', so that modules (like the Book-E
watchdog driver) can use it.  To maintain consistency, ppc_proc_freq is
changed to a GPL-only export.  This is okay, because any module that needs
this symbol should be an actual Linux driver, which must be GPL-licensed.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>