mfd: as3722: Convert MFD_AS3722 from bool to tristate

The Kconfig currently controlling compilation of this code is:

drivers/mfd/Kconfig:config MFD_AS3722
drivers/mfd/Kconfig:    bool "ams AS3722 Power Management IC"

...meaning that it currently is not being built as a module by anyone.

In response to an earlier patch set suggesting removal of the unused
modular code, Laxman suggested that this driver be instead moved to
tristate.

We do that here, and confirm it can build and modpost as a tristate.
However there remains to be runtime testing in order to ensure this
change is 100% functional for "=m".

Cc: Florian Lobmaier <florian.lobmaier@ams.com>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>