spl: Provide more space to be used for storing SPL on imx28 OCRAM

With the current configuration provided in mxsimage{-spl}.mx28.cfg the
size of SPL binary has been constrained to 32 KiB, due to "LOAD IVT"
command with 0x8000 offset.

The problem was that, the imx28 ROM takes the u-boot.sb and then
extracts from it the IVT header and places it on the 0x8000 OCRAM offset
overwriting any valid (i.e. loaded from eMMC or SPI-NOR) SPL code. This
bug was unnoticed as the overwrite size was just 32 bytes, so the
probability that some important code is altered was low.

However, in the XEA board (where the SPL size is ~39KiB), the overwritten
data was `(struct dm_spi_ops *) 0x800c <mxs_spi_ops>`, which is used
during the boot process.

As a result the SPL execution code hanged with "undefined instruction"
abort as callbacks (with wrong addresses) from it were called.

The fix is to change the OCRAM's offset where IVT is loaded to 0xE000,
so the SPL can grow up to ~57KiB (the maximal size of OCRAM memory
available is 0xE3FC).

Signed-off-by: Lukasz Majewski <lukma@denx.de>

arch/arm/cpu/arm926ejs/mxs/mxsimage-spl.mx28.cfg

@@ -2,5 +2,5 @@ DISPLAYPROGRESS
 SECTION 0x0 BOOTABLE
  TAG LAST
  LOAD     0x1000     spl/u-boot-spl.bin
- LOAD IVT 0x8000     0x1000
- CALL HAB 0x8000     0x0
+ LOAD IVT 0xE000     0x1000
+ CALL HAB 0xE000     0x0

arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg

@@ -2,8 +2,8 @@ DISPLAYPROGRESS
 SECTION 0x0 BOOTABLE
  TAG LAST
  LOAD     0x1000     spl/u-boot-spl.bin
- LOAD IVT 0x8000     0x1000
- CALL HAB 0x8000     0x0
+ LOAD IVT 0xE000     0x1000
+ CALL HAB 0xE000     0x0
  LOAD     0x40002000 u-boot.bin
- LOAD IVT 0x8000     0x40002000
- CALL HAB 0x8000     0x0
+ LOAD IVT 0xE000     0x40002000
+ CALL HAB 0xE000     0x0