RPI: Fix DMA/SDHCI on the BCM2836 (Raspberry Pi 2)

r354875 pushed VCBUS <-> ARMC translations to runtime determination, but
incorrectly mapped addresses for the BCM2836 -- SOC_BCM2835 and SOC_BCM2836
are actually mutually exclusive, so the BCM2836 config (GENERIC) would have
taken the latter path in the header and used 0x3f000000 as peripheral start.

Easily fixed -- split out the BCM2836 into its own memmap config and use
that instead if SOC_BCM2836 is included. With this, we get back to userland
again.

Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>