target/m68k: rename M68K_FEATURE_M68000 to M68K_FEATURE_M68K

The M68K_FEATURE_M68000 feature is misleading in that its name suggests the feature
is defined just for Motorola 68000 CPUs, whilst in fact it is defined for all
Motorola 680X0 CPUs.

In order to avoid confusion with the other M68K_FEATURE_M680X0 constants which
define the features available for specific Motorola CPU models, rename
M68K_FEATURE_M68000 to M68K_FEATURE_M68K and add comments to clarify its usage.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220917112515.83905-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>