Limit low-latency changes in SDHCI driver only to BCM2708 build because they break RPi DTS build.

The DTS enabled kenel build selects CONFIG_ARCH_MULTIPLATFORM and CONFIG_ARCH_BCM2835
options. This leads to use of use option and SDHCI variant CONFIG_MMC_SDHCI_BCM2835,
because CONFIG_MMC_SDHCI_BCM2708 is tied to MACH_BCM2708 selection.

This change limits next instrusive BCM2708 change only respective kernel
build

  Add low-latency mode to sdcard driver. Disable with sdhci-bcm2708.enable_llm=0.
Add "brcm,bcm2835-usb" OF alias to DesignWare DW2 driver to match device tree from recent kernels.

With this change even older driver found in 3.12 tree is can be used
with devicetree generated from actual kernel sources.

DWC2 requires coherent memory poll to work with LAN9500 USB 2.0
to Ethernet 10/100 Adapter in this configuration and default
coherent pool size (256kB) is too small to satisfy requirements.
This can be enlarged by next kernel option

  coherent_pool=2M