Add a spi_private.h with a SPI_DEV(), for convenience. Use it in the
F1 and F2 implementations. We could probably unify these with an
STM32_HAVE_SPI(n) macro, but we'll leave that for the future.
Most everything from F1 is portable; F2 has some additional bit
definitions and a spi_get_af() routine, but that's about it.
Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
