FROMLIST: arm64: Enable CONFIG_ARM64_SW_TTBR0_PAN

This patch adds the Kconfig option to enable support for TTBR0 PAN
emulation. The option is default off because of a slight performance hit
when enabled, caused by the additional TTBR0_EL1 switching during user
access operations or exception entry/exit code.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 31432001
Change-Id: Id00a8ad4169d6eb6176c468d953436eb4ae887ae
(cherry picked from commit 6a2d7bad43474c48b68394d455b84a16b7d7dc3f)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>