optee: Add optee_image_get_entry_point()

Add a helper function for extracting the least significant 32 bits from the
OPTEE entry point address, which will be good enough to load OPTEE binaries
up to (2^32)-1 bytes.

We may need to extend this out later on but for now (2^32)-1 should be
fine.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Harinarayan Bhatta <harinarayan@ti.com>
Cc: Andrew F. Davis <afd@ti.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Peng Fan <peng.fan@nxp.com>
Tested-by: Peng Fan <peng.fan@nxp.com>