Added support for multiple SD cards

The MMCFS implementation can now handle more than one MMC card. To
use this feature, define these macros in platform_conf.h:

MMCFS_NUM_CARDS - the total number of cards
MMCFS_CS_PORT_ARRAY - array of CS ports
MMCFS_CS_PIN_ARRAY - array of CS pins
MMCFS_SPI_NUM_ARRAY - array of SPI port numbers

For example:
 #define MMCFS_NUM_CARDS          2
 #define MMCFS_CS_PORT_ARRAY      { 7, 2 }
 #define MMCFS_CS_PIN_ARRAY       { 0, 5 }
 #define MMCFS_SPI_NUM_ARRAY      { 0, 0 }

defines a system with 2 MMC cards. The first one uses SPI0 and has its
CS on PG0. The second one also uses SPI0 (this is perfectly possible)
but has its CS on PC5. This was the configuration used to test this
feature on a EK-LM3S8962 board.
Style changes

- remove stray printf
- use a consistent naming scheme for MMCFS_NUM_CARDS
updated documentation about multiple SD card support