arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry

arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry

CVE-2017-5754 ARM64 KPTI fixes

Although CONFIG_UNMAP_KERNEL_AT_EL0 does make KASLR more robust, it's
actually more useful as a mitigation against speculation attacks that
can leak arbitrary kernel data to userspace through speculation.

Reword the Kconfig help message to reflect this, and make the option
depend on EXPERT so that it is on by default for the majority of users.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 0617052ddde355ee663b2f048e67dd381e5ebd6a)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>