arm64: Disable EL2 traps for BRBE instructions executed in EL1

This disables EL2 traps for BRBE instructions executed in EL1. This would
enable BRBE to be configured and used successfully in the guest kernel.
While here, this updates Documentation/arm64/booting.rst as well.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>