mmc: sdhci-pltfm: Do not use parent as the host's device

The code selecting a device for the sdhci host has been
continuously tweaked (4b711cb1
"mmc: sdhci-pltfm: Add structure for host-specific data" and
a4d2177f "mmc: sdhci-pltfm: dt
device does not pass parent to sdhci_alloc_host" while there
does not seem to be any reason to use platform device's parent
in the first place.

The comment saying "Some PCI-based MFD need the parent here"
seem to refer to Timberdale FPGA driver (the only MFD driver
registering SDHCI cell, drivers/mfd/timberdale.c) but again,
the only situation when parent device matter is runtime PM,
which is not implemented for Timberdale.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>