mmc: host: omap_hsmmc: Fix MMC for omap3 legacy booting

Starting with commit 7d607f91 ("mmc: host: omap_hsmmc: use
devm_regulator_get_optional() for vmmc") MMC on omap3 stopped working
for legacy booting.

This is because legacy booting sets up some of the resource in the
platform init code, and for optional regulators always seem to
return -EPROBE_DEFER for the legacy booting.

Let's fix the issue by checking for device tree based booting for
now. Then when omap3 boots in device tree only mode, this patch
can be just reverted.

Fixes: 7d607f91 ("mmc: host: omap_hsmmc: use
devm_regulator_get_optional() for vmmc")
Cc: Felipe Balbi <balbi@ti.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>