arm64: Make 36-bit VA depend on EXPERT

Commit 21539939 (arm64: 36 bit VA) introduced 36-bit VA support for
the arm64 kernel when the 16KB page configuration is enabled. While this
is a valid hardware configuration, it's not something we want to
encourage since it reduces the memory (and I/O) range that the kernel
can access. Make this depend on EXPERT to avoid complaints of Linux not
mapping the whole RAM, especially on platforms following the ARM
recommended memory map.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>