arm: mvebu: Add missing "if SPL"

We can only select SPL_SKIP_LOWLEVEL_INIT if SPL is enabled, otherwise
we get a warning about unmet dependencies on platforms that don't use
SPL.

Fixes: cf47a8cf ("arm: mvebu: Select SPL_SKIP_LOWLEVEL_INIT on ARMADA_32BIT")
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>