arm: set default vector table address in prep_c when XIP

It is called before early SoC initialization, so remove the duplicated
code from other boards and just set it by default when using XIP.

This can later be used when adding bootloader support, as an
additional option could be created to move the VTOR offset to a
different address.

Change-Id: Ia1f5d9a066de61858ee287215cefdd58596b6b1c
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>