[POWERPC] FSL: fix mpc83xx_spi device registration

calling platform_device_register after platform_device_alloc causes
this:

kobject (c3841a70): tried to init an initialized object, something is seriously wrong.
Call Trace:
[c381fe20] [c0007bb8] show_stack+0x3c/0x194 (unreliable)
[c381fe50] [c01322a8] kobject_init+0xb8/0xbc
[c381fe60] [c01591cc] device_initialize+0x30/0x9c
[c381fe80] [c015ee34] platform_device_register+0x1c/0x34
[c381fea0] [c02f1fe0] of_fsl_spi_probe+0x21c/0x22c
[c381ff30] [c02f2044] fsl_spi_init+0x54/0x160
[c381ff60] [c02f3924] __machine_initcall_mpc832x_rdb_mpc832x_spi_init+0x120/0x138
[c381ff70] [c02e61b4] kernel_init+0x98/0x284
[c381fff0] [c000f740] kernel_thread+0x44/0x60

fixed by calling platform_device_add (second half of
platform_device_register) instead.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>