i.MX28: Fix initial stack pointer position

i.MX28: Fix initial stack pointer position

The patch:

  m28evk: Use GENERATED_GBL_DATA_SIZE
  commit 1084606c972ea5f1d89f69bdbd978b867d0ee521

introduced usage of GENERATED_GBL_DATA_SIZE and calculation of initial stack
pointer position defived from that. Due to a small typo, the SP position moved
to 0x21f80, which is past the SRAM area. This didn't manifest on the real
hardware as the SRAM repeats there (address bits in the CPU being ignored).

Though this was caught in QEMU, where it crashed the emulator.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>