[U-Boot] 64Bit device tree compilation

[Mark Rutland]

Hi,

> Maybe a dumb question, why do we need to have a 64-bit U-Boot for
> arm64? I don't see we ever created 64-bit U-Boot for ppc64.

In ARMv8 it's not possible to change the register width at an exception
level (i.e. you can't change 64->32 or vice-versa), and lower exception
levels cannot be wider (so if your code at EL3 is 32-bit, you cannot run
64-bit code at EL3, EL2, EL1, or EL0).

Therefore you need a purely 64-bit path from EL3 to EL2 or EL1N in order
to boot a 64-bit kernel, so the bootloader needs to be 64-bit.

Mark.