[PATCH RFC] Make BCMA a menuconfig to ease disabling it all

[Vincent Legoll]

Hello,

this is more what I wanted to achieve, an easily disableable BCMA menuconfig.

This is working as intended on x86_64 defconfig, whereas it is harder to disable
for ARCH=arm because of "select"s. But that's less of an issue, as those arch's
defconfigs may very well want the feature enbled, and that would make sense.

Any input appreciated.

Thanks