mfd: axp20x-i2c: Document that this must be builtin on x86

On x86 the AXP288 PMIC provides an ACPI OpRegion handler, which must be
available before other drivers using it are loaded, which can only be
ensured if the MFD, OpRegion and i2c-bus drivers are built-in.

Since the AXP20x MFD code is used on non x86 too we cannot simply change
this into a bool, I've tried some Kconfig magic with if x86 but I could
not get this working correctly, so this commit just documents that this
should be built-in on x86, which fixes errors like these during boot:

mmc0: SDHCI controller on ACPI [80860F14:00] using ADMA
ACPI Error: No handler for Region [REGS] (ffff93543b0cc3a8) [UserDefinedRegion]
ACPI Error: Region UserDefinedRegion (ID=143) has no handler (20170119/exfldio-2
ACPI Error: Method parse/execution failed [\_SB.PCI0.I2C7.PMI5.GET] (Node ffff93
ACPI Error: Method parse/execution failed [\_SB.PCI0.SHC1._PS0] (Node ffff93543b
acpi 80860F14:02: Failed to change power state to D0

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: russianneuromancer <russianneuromancer@ya.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>