mfd: qcom-pm8xxx: Clean up PM8XXX namespace

The Kconfig and file naming for the PM8xxx driver is totally
confusing:

- Kconfig options MFD_PM8XXX and MFD_PM8921_CORE, some in-kernel
  users depending on or selecting either at random.
- A driver file named pm8921-core.c even if it is indeed
  used by the whole PM8xxx family of chips.
- An irqchip named pm8xxx since it was (I guess) realized that
  the driver was generic for all pm8xxx PMICs.

As I may want to add support for PM8901 this is starting to get
really messy. Fix this situation by:

- Remove the MFD_PM8921_CORE symbol and rely solely on MFD_PM8XXX
  and convert all users, including LEDs Kconfig and ARM defconfigs
  for qcom and multi_v7 to use that single symbol.
- Renaming the driver to qcom-pm8xxx.c to fit along the two
  other qcom* prefixed drivers.
- Rename functions withing the driver from 8921 to 8xxx to
  indicate it is generic.
- Just drop the =m config from the pxa_defconfig, I have no clue
  why it is even there, it is not a Qualcomm platform. (Possibly
  older Kconfig noise from saveconfig.)

Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>