When building the kernel with CONFIG_THUMB2_KERNEL enabled, older
assemblers may emit the following error:

reset-handler.S:78: Error: invalid immediate for address calculation (value = 0x00000004)

Using an explicit adr.w instruction will solve this. Newer assemblers do
this automatically. Use the W() macro to do this under Thumb mode only.
Inspired-by: Joseph Lo <josephl@nvidia.com>
Suggested-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>