m68k: Define NR_CPUS to 1

m68k: Define NR_CPUS to 1

This fixes a Kconfig warning

	fs/erofs/Kconfig:137:warning: range is invalid

which originates from EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS using
NR_CPUS which up to now didn't exist for ARCH=m68k.  All other
architectures define this symbol, so fix the outlier.

[geert] This also fixes:
  - CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS being set to the
    literal NR_CPUS instead of a number by automatic configs like
    "make allmodconfig" or "make olddefconfig",
  - An infinite loop in manual configs like "make oldconfig" when
    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS is not present or
    has an invalid value in your existing .config.

Fixes: c9b47e6b2311 ("erofs: cap LZMA stream pool size")
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patch.msgid.link/20260731094950.1988084-2-ukleinek@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>