powerpc/cpm: Reintroduce global spi_pram struct (fixes build issue)

spi_t was removed in commit 644b2a68


("powerpc/cpm: Remove SPI defines and spi structs"), the commit assumed
that spi_t isn't used anywhere outside of the spi_mpc8xxx driver. But
it appears that the struct is needed for micropatch code. So, let's
reintroduce the struct.

Fixes the following build issue:

    CC      arch/powerpc/sysdev/micropatch.o
  micropatch.c: In function 'cpm_load_patch':
  micropatch.c:629: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
  micropatch.c:629: error: 'spp' undeclared (first use in this function)
  micropatch.c:629: error: (Each undeclared identifier is reported only once
  micropatch.c:629: error: for each function it appears in.)

Reported-by: LEROY Christophe <christophe.leroy@c-s.fr>
Reported-by: Tony Breeds <tony@bakeyournoodle.com>
Cc: <stable@kernel.org> [ .33, .34 ]
Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>