arm: Kconfig: Explain TFABOOT

The CONFIG_TFABOOT option is more about what U-Boot DOES NOT need to do
than to support some features.

Explain a bit more in the Kconfig help text to avoid misunderstandings.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>