bsps spi naming conventions

[Jacob Rosenthal]

In updating a bunch of bsps I found most of the nrf51 devices create a
spi_0_master and spi_1_slave but nrf52 boards seem to create
an spi_0_master  and spi_0_slave

Shall we standardize on one or the other? Does the first slave start at 0
or 1?