ARM: debug: fix DEBUG_UNCOMPRESS help for !MULTIPLATFORM

Commit 84fc8636

 ("ARM: make ARCH_MULTIPLATFORM user-visible") modified
DEBUG_UNCOMPRESS to prevent using it with multiplatform kernels.

Update the help text, remove references to multiplatform.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>