powerpc/83xx: sbc8349 - update defconfig, enable MTD, USB storage

With flash partition entries in the DTS file, MTD might as well
be enabled in the defconfig.  In a similar vein, enable USB and
enough related options (SCSI/ext2/ext3) so that a user can read
and write to a generic USB flash drive as well.

Also, this board only has the two default SOC UARTs, so adjust the
UART config accordingly.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>