When building linux-on-litex-vexriscv for OrangeCrab:

    Warning: IOBUF 'spisdcard_clk' attribute 'SLEW' is not recognised (on line 207)
    Warning: IOBUF 'spisdcard_mosi' attribute 'SLEW' is not recognised (on line 210)
    Warning: IOBUF 'spisdcard_cs_n' attribute 'SLEW' is not recognised (on line 214)
    Warning: IOBUF 'spisdcard_miso' attribute 'SLEW' is not recognised (on line 218)

Platforms using litex.build.lattice.LatticePlatform seem to support only
"SLEWRATE", not "SLEW".  Fix the few offenders in the LogicBone and
OrangeCrab platform definitions.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>