spi: Tolerate platforms defining their enums differently

[chrysn]

When testing them with full RIOT, #141 had errors like this:

100 |             clk: riot_sys::spi_clk_t_SPI_CLK_5MHZ,
    |                            ^^^^^^^^^^^^^^^^^^^^^^ not found in `riot_sys`

This PR uses the cfg(accessible()) emulation to switch around between how the value is best accessed.

[chrysn]

Testing is a bit odd because we can't rely on CI (after all, #141 passed) and I don't want to add just another board for this PR. I tested this by running in tests/spi the command RIOTBASE=~/git/RIOT make BOARD=stm32f429i-disc1 -- a build test suffices here.

[chrysn]

Just Also tested with make buildtest BOARDS="remote-revb maple-mini cc2538dk samr34-xpro stm32f429i-disco stm32l496g-disco", an arbitrary selection just to get a bit more breadth.

I'm setting this to draft, tough, because on BOARD=usb-kw41z, I found more oddities; maybe this belongs in here still.

[Teufelchen1]

LGTM but not sure if I can assess the in-depth implications of this.