stm32mp: compute ram_top based on the optee base address

The value of ram_top address currently gets computed in an indirect
manner. The boot_fdt_add_mem_rsv_regions() function gets called first
to reserve the memory region occupied by OP-TEE in the LMB memory
map. This is followed by a call to the lmb_alloc() API, which returns
an address which is below the OP-TEE base address. This address is the
value of ram_top returned by the board_get_usable_ram_top() function.

This has now changed, as the LMB memory map, which is no longer local,
gets set up after relocation. Get the OP-TEE base address by reading
the device tree, and set the ram_top from this value.

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