omap3 beagle: nand (mostly) cleanups

Clean up the Beagle NAND config a bit:  use SZ_128K instead of
an arithmetic expression for the NAND block size.  Set the
NAND_BUSWIDTH_16 flag.  Make its init routine static.

(Unrelated:  make omap3_beagle_i2c_init be part of beagle_init.
It shouldn't be called on non-beagle hardware; and beagle_init
already runs with other arch_initcall code.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Richard Woodruff <r-woodruff2@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>