arm: Remove 32bit KVM host support

That's it. Remove all references to KVM itself, and document
that although it is no more, the ABI between SVC and HYP still
exists.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>