drivers/i2c/omap24xx_i2c.c: move all local variables to SRAM

At old overo boards TWL4030 RTC irq is connected to gpio112. Unfortunately
this pin is also used for revision detection. Therefore we need to send
shut-up to TWL4030 to avoid reading wrong revision. In SPL this must
be done before SDRAM is set up because the type of SDRAM is revision dependent.
By this patch it is ensured that all variables used by omap24xx_i2c.c are
located in SRAM.

Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Acked-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>