arm/dt: msm8974: Switch to SDHCI by default

The Secure Digital Controller in msm8974 supports both legacy
(ARM PrimeCell MMCI PL180 interface) mode and Secure Digital
Host Controller Interface.  Switch to using the latter.

CRs-Fixed: 462126
Change-Id: I2d7d21f49d8bf817407e2ac665d0592605e5b9fe
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>