lib: Make prime number generator independently selectable

Make prime number generator independently selectable from
kconfig. This allows us to enable CONFIG_PRIME_NUMBERS=m
and run the tools/testing/selftests/lib/prime_numbers.sh
without other DRM selftest modules.

Signed-off-by: Masami Hiramatsu <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Luis Chamberlain <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/math/Kconfig

···
 	  calculations are in fixed point. Module will be called cordic.
 
 config PRIME_NUMBERS
-	tristate
+	tristate "Simple prime number generator for testing"
+	help
+	  This option provides a simple prime number generator for test
+	  modules.
+
+	  If unsure, say N.
 
 config RATIONAL
 	bool