plat: marvell: armada: a3k: When WTP is empty do not define variables and targets which depends on it

Some of targets (e.g. mrvl_flash) depends on WTP build option. Other
targets (e.g. fip) can be build also without WTP build option as they do
not depend on it.

This change put all A3720 variables and targets which depends on WTP into
conditional if-endif section, so they are not defined when user has not
supplied WTP build option.

Target mrvl_flash is defined also when WTP was not specified and in this
case it just print error message to help user.

Variables which do not depend on WTP are moved to the top of
a3700_common.mk file.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: Idb3892233586a0afca3e0e6564279641d2e4b960