ARM: fixed relocation using proper alignment

Using u-boot-2017.05 on i.MX6UL we ran into following problem:
Initially U-Boot could be started normally.
If we added one random command in configuration, the newly generated
image hung at startup (last output was DRAM:  256 MiB).

We tracked this down to a data abort within relocation (relocated_code).

relocated_code in arch/arm/lib/relocate.S copies 8 bytes per loop
iteration until the source pointer is equal to __image_copy_end.
In a good case __image_copy_end was aligned to 8 bytes, so the loop
stopped as suggested, but in an errornous case __image_copy_end was
not aligned to 8 bytes, so the loop ran out of bounds and caused a
data abort exception.

This patches solves the issue by aligning __image_copy_end to 8 byte
using the linker script related to arm.

I don't know if it's the correct way to solve this, so some review would
be very appreciated.