Rename `spin.h` -> `boot.h`

In `spin.h` we have function prototypes provided by `boot.c`. For
clarity, let's align the naming.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
3 files changed