optee: adjust dependencies and default values for dram

We may have, the not yet considered, scenario where OPTEE is loaded before
u-boot and *not* by u-boot, e.g, the boot flow using the ARM Trusted
Firmware (ATF), where in the 32bit flow is:
BootRom->ATF(BL2)->Optee(BL32)->u-boot(BL33)

In this case we need still to reserve the memory used by optee, to avoid
for example to realocate ourself to the same address at the end of DRAM.
So, we change here the dependencies on the OPTEE lib and we set the default
size and base of TZRAM to zero.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Cc: u-boot@lists.denx.de