I just started looking into this today. Check hsmmc.c in the arch/arm/
mach-omap2/ directory. Add a second resource for the hsmmc2
controller, and add new functions for power, card detect, etc. I just
stubbed them out and added some printk's. After changing the pinmux
config in u-boot, and hooking a logic analyzer up to MMC2 clk and cmd
lines, I can see the SD stack issuing a CMD0 on the SD bus. I haven't
hooked a card up yet, but I expect it will work fine when I do.
I've also been trying to get UART2 working with the B6 board. I see
RTS go low when I cat a text file to /dev/ttyS1, but no activity on
the TX line. Looks like the recent u-boot (from git) pinmux is
already set up properly.