ARM: omap3: Add CONFIG_SPL_BOARD_INIT for CONFIG_SPL_MMC_SUPPORT

As originally reported against beagleboard we currently have the
following error message reported in SPL:

U-Boot SPL 2012.04-00020-gb8310b9-dirty (Apr 25 2012 - 18:49:57)
Texas Instruments Revision detection unimplemented
OMAP SD/MMC: 0
timed out in wait_for_bb: I2C_STAT=1000
reading u-boot.img
....

The reason for above message is that when booting from MMC, I2C needs to
be initialized to talk with the TWL4030.  On OMAP3 I2C is only
initalized in SPL if CONFIG_SPL_BOARD_INIT is set.

Cc: Thomas Weber <weber@corscience.de>
Cc: Steve Sakoman <sakoman@gmail.com>

Original patch for Beagleboard is:
Signed-off-by: Peter Meerwald <p.meerwald@bct-electronic.com>

Extended to cover all other boards:
Signed-off-by: Tom Rini <trini@ti.com>