mx51evk: support new relocation scheme

This patch is to fix build breakage and support new relocation
scheme for mx51evk.

- Correct IRAM base address and add size definition

  The IRAM starts from 0x1FFE0000 on final revsion i.mx51 than
  0x1FFE8000 which is for older revision.

- Include imx-regs.h in mx51evk.h

  Definitions like CSD0_BASE_ADDR and IRAM_BASE_ADDR can be
  referred to.

- Define CONFIG_SYS_INIT_RAM_ADDR and CONFIG_SYS_INIT_RAM_SIZE

  They are used to define init RAM layout.

- Remove comment for CONFIG_SYS_GBL_DATA_SIZE which has been
  buried by Wolfgang's commit below

  25ddd1fb: Replace CONFIG_SYS_GBL_DATA_SIZE by auto-generated value

Signed-off-by: Shawn Guo <shawn.gsc@gmail.com>