arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry

arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry

commit 0617052ddde3 upstream.

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>
Signed-off-by: Alex Shi <alex.shi@linaro.org>