mfd: MFD_SL28CPLD should depend on ARCH_LAYERSCAPE

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: a538ad22 ("mfd: simple-mfd-i2c: Add sl28cpld support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Michael Walle <michael@walle.cc>
Signed-off-by: Lee Jones <lee.jones@linaro.org>