optee: Make TZDRAM config options contingent on CONFIG_OPTEE

Commit c7b3a7ee ("optee: adjust dependencies and default values for
dram") makes the TZDRAM defines for OPTEE show up for all configs as a
side-effect. While not harmful its not what we really want.

This patch makes the following defines contingent on CONFIG_OPTEE=y

CONFIG_OPTEE_TZDRAM_BASE
CONFIG_OPTEE_TZDRAM_SIZE

Rightly, if you don't have CONFIG_OPTEE=y you don't care about the above
two defines.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Rui Miguel Silva <rui.silva@linaro.org>
Acked-by: Rui Miguel Silva <rui.silva@linaro.org>