bootcount: spl: Enable bootcount support in SPL

New, SPL related config option - CONFIG_SPL_BOOTCOUNT_LIMIT has been
added to allow drivers/bootcount code re-usage in SPL.

This code is necessary to use and setup bootcount in SPL in the case of
falcon boot mode.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Alex Kiernan <alex.kiernan@gmail.com>