driver/mxc_i2c: Move static data structure to global_data

This driver needs a data structure in SRAM before SDRAM is available.
This is not alway the case using .data section. Moving this data
structure to global_data guarantees it is writable.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Troy Kisky <troy.kisky@boundarydevices.com>