Currently the Kontron sl28cpld Board Management Controller is found only
on Kontron boards equipped with a Freescale Layerscape SoC. Hence add a
dependency on ARCH_LAYERSCAPE, to prevent asking the user about a driver
for this controller when configuring a kernel without Layerscape support.
Fixes: a538ad229bbee4f8 ("mfd: simple-mfd-i2c: Add sl28cpld support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>