warp7: Specify a default CONFIG_OPTEE_LOAD_ADDR if non provided

If no CONFIG_OPTEE_LOAD_ADDR is provided i.e. you are not loading OPTEE
into memory in u-boot, then just set the non-existent CONFIG option to
zero, elsewise stringify(CONFIG_OPTEE_LOAD_ADDR) will return
"CONFIG_OPTEE_LOAD_ADDR" - which looks weird in the u-boot environment.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>