New optimized mmc driver for the Zipit. 

The mmc.c file is a drop in replacement for drivers/mmc/mmc.c in Ken's
kernel tree.

WARNING: backup your mmc card data before testing this new driver!

The mmc.o driver makes multi-block reads to the card.  If your card does
not support such reads, the mmc-nomulti.o driver can be used with only a
slight decrease in performance.

Performance:
  original driver:  120kiB/sec read/write
  optimized mmc.o:  220kiB/sec read, 240kiB/sec write
  mmc-nomulti.o:    200kiB/sec read, 240kiB/sec write