mx25: Fix boot hang by avoiding vector relocation

Since commit 3ff46cc4 ("arm: relocate the exception vectors") mx25pdk
hangs like this:

CPU:   Freescale i.MX25 rev1.2 at 399 MHz
Reset cause: WDOG
Board: MX25PDK
I2C:   ready
DRAM:  64 MiB
(hangs)

Add a specific relocate_vectors macro that skips the vector relocation, as the
i.MX25 SoC does not provide RAM at the high vectors address (0xFFFF0000), and
(0x00000000) maps to ROM.

This allows mx25 to boot again.

Acked-By: Bill Pringlemeir <bpringlemeir@nbsps.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>