arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry

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: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2056d452aa0015384a5b09a67b0a22e548b42e1e
Git-Commit: 2897537b
Git-repo: git://android.googlesource.com/kernel/common.git


[vinmenon@codeaurora.org: trivial merge conflicts]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>