mfd: remove CONFIG_MFD_SUPPORT

We currently have two symbols to control compilation the MFD subsystem,
MFD_SUPPORT and MFD_CORE. The MFD_SUPPORT is actually not required
at all, it only hides the submenu when not set, with the effect that
Kconfig warns about missing dependencies when another driver selects
an MFD driver while MFD_SUPPORT is disabled. Turning the MFD submenu
back from menuconfig into a plain menu simplifies the Kconfig syntax
for those kinds of users and avoids the surprise when the menu
suddenly appears because another driver was enabled that selects this
symbol.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>