mips: If CONFIG_CMD_SPI is defined, call spi_init()

The mips architecture currently does not call 'spi_init()' in the generic
board initialization routine is CONFIG_CMD_SPI is defined.

This patch rectifies that problem.

Signed-off-by: Jason McMullan <mcmullan@netapp.com>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>