x86/cpu: Document CONFIG_X86_INTEL_MID as 64-bit-only

The X86_INTEL_MID code was originally introduced for the 32-bit
Moorestown/Medfield/Clovertrail platform, later the 64-bit
Merrifield/Moorefield variants were added, but the final Morganfield
14nm platform was canceled before it hit the market.

To help users understand what the option actually refers to, update the
help text, and add a dependency on 64-bit kernels.

Ferry confirmed that all the hardware can run 64-bit kernels these days,
but is still testing 32-bit kernels on the Intel Edison board, so this
remains possible, but is guarded by a CONFIG_EXPERT dependency now,
to gently push remaining users towards using CONFIG_64BIT.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Shevchenko <andy@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250226213714.4040853-9-arnd@kernel.org