spl: bootcount: Move code out of header file

spl: bootcount: Move code out of header file

It is not good practice to write code in a header file. If it is included
multiple times then the code can cause duplicate functions.

Move the bootcount_store() and bootcount_load() functions into SPL.

Note: bootcount is a bit strange in that it uses driver model but does not
define proper drivers. This should be fixed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>