powerpc/85xx: Make eSDHC 1-bit only transfer mode default for MPC8569E-MDS

For yet unknown reason 4-bit mode doesn't work on MPC8569E-MDS boards,
so make 1-bit mode default. When we resolve the issue, u-boot will
remove sdhci,1-bit-only property from the device tree, while SDHCI
will still work with older u-boots.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>