arm64: head: create a new function for setting the boot_cpu_mode flag

arm64: head: create a new function for setting the boot_cpu_mode flag

Currently, the code for setting the __cpu_boot_mode flag is munged in
with el2_setup. This makes things difficult on a BE bringup as a
memory access has to have occurred before el2_setup which is the place
that we'd like to set the endianess on the current EL.

Create a new function for setting __cpu_boot_mode and have el2_setup
return the mode the CPU. Also define a new constant in virt.h,
BOOT_CPU_MODE_EL1, for readability.

Acked-by: Marc Zyngier <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Matthew Leach <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>