ARM: bcm2835: add SDHCI node to DT

Add the SDHCI device node to the SoC DT file. Add a dummy fixed-clock
to satisfy the SDHCI driver's clock lookup; eventually this should be
replaced by a real clock implementation. Add board specific properties
to the Raspberry Pi board file.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>