spl: add option to disable SPL banner output

Selecting this option will reduce SPL boot time by approx. 6 ms
(e. g. with 70 bytes long banner string at 115200 baud).

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>