mfd: max77693: Allow building as a module

The consumer of max77693 regulators on Trats2 board (samsung-usb2-phy
driver) supports deferred probing so the max77693 main MFD driver can be
built now as a module. This gives more flexibility and removes manual
ordering of init calls.

Suggested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>