powerpc/mpic: remove unused functions

Drop unused fsl_mpic_primary_get_version(), mpic_set_clk_ratio(),
mpic_set_serial_int().

  + fsl_mpic_primary_get_version() is just a safe wrapper around
fsl_mpic_get_version() for SMP configurations. While the latter is
called explicitly for handling PIC initialization and setting up error
interrupt vector depending on PIC hardware version, the former isn't
used for anything.

  + As for mpic_set_clk_ratio() and mpic_set_serial_int(), they both are
almost nine years old[1] but still have no chance to be called even from
out-of-tree modules because they both are __init and of course aren't
exported.

[1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2006-June/023867.html

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Cc: hongtao.jia@freescale.com
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>