arm64: kexec: always reset to EL2 if present

Currently machine_kexec() doesn't reset to EL2 in the case of a
crashdump kernel. This leaves potentially dodgy state active at EL2, and
means that if the crashdump kernel attempts to online secondary CPUs,
these will be booted as mismatched ELs.

Let's reset to EL2, as we do in all other cases, and simplify things. If
EL2 state is corrupt, things are already sufficiently bad that kdump is
unlikely to work, and it's best-effort regardless.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>