arm: K3: Update _start instruction

On K3 family SoCs, once the ROM loads image on R5, M3 resets R5 and
expects to start executing from 0x0. In order to handle this ROM
updates the boot vector of R5 such that first 64 bytes of image load
address are mapped to 0x0.

In this case, it is SPL's responsibility to jump to the proper image
location. So, update the PC with address of reset vector(like how
other exception vectors are handled), instead of branching to reset.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>