stm32mp: allow calling optee_get_reserved_memory() from U-Boot

The optee_get_reserved_memory() function returns the OP-TEE base
address and size. The function gets these values from the
FDT. Currently, this function is defined only to be called in the SPL
phase. Move this function to a place where it can be invoked from the
main U-Boot phase, where it will be used to compute the ram_top
address.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>