i2c: omap2+: change header guard to be generic

Make the header guard to be generic to stop conflicting with
omap2 i2c header file arch/arm/include/asm/arch-omap24xx/i2c.h

Cc: Steve Sakoman <steve@sakoman.com>
Cc: Heiko <hs@denx.de>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Cc: Wolfang Denk <wd@denx.de>

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Steve Sakoman <steve@sakoman.com>