If it's all their code, yes they can do it (it might be a lot of engineering, though). But it might contain 3rd party code or dependencies to something that's non trivial to port to 64 bits.
If you go look at the boot procedure of a modern ARM SoC, it's pretty complex (also it's different for every SoC, there's no governing standard like the "PC is for x86"). The CPU may go through several operating modes and the procedure of booting up the cores is non-trivial.
I do not think it's impossible, but the benefits for all that engineering work may not be as great as GP suggests.
They are in a pretty unique position as they control all the hardware and the software, though.
Actually there are more or less only two big ones used in 2017: the well engineered open source boot solution that ARM started and is now backed by Linux and that piece of garbage that Qualcomm uses.
Hopefully next year this time there will be only one.
