arm64: booting.rst: Clarify on requiring non-secure EL2

The ARMv8.4 architecture revision introduced the EL2 exception level
to the secure world. Clarify the existing wording to make sure that
Linux relies on being executed in the non-secure state.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220107160056.322141-2-andre.przywara@arm.com


Signed-off-by: Will Deacon <will@kernel.org>