soc: sunxi: select CONFIG_PM

Selecting CONFIG_PM_GENERIC_DOMAINS without CONFIG_PM leads to a
build failure:

WARNING: unmet direct dependencies detected for PM_GENERIC_DOMAINS
  Depends on [n]: PM [=n]
  Selected by [y]:
  - SUN20I_PPU [=y] && (ARCH_SUNXI [=n] || COMPILE_TEST [=y])

drivers/base/power/domain_governor.c: In function 'default_suspend_ok':
drivers/base/power/domain_governor.c:85:24: error: 'struct dev_pm_info' has no member named 'ignore_children'
   85 |         if (!dev->power.ignore_children)
      |                        ^
drivers/base/power/domain.c: In function 'genpd_queue_power_off_work':
drivers/base/power/domain.c:657:20: error: 'pm_wq' undeclared (first use in this function)
  657 |         queue_work(pm_wq, &genpd->power_off_work);
      |                    ^~~~~

Unfortunately platforms are inconsistent between using 'select PM'
and 'depends on PM' here. CONFIG_PM is a user-visible symbol, so
in principle we should be using 'depends on', but on the other hand
using 'select' here is more common among drivers/soc. Go with the
majority for now, as this has a smaller risk of introducing circular
dependencies. We may need to clean this up for consistency later.

Fixes: 0e30ca5a ("soc: sunxi: Add Allwinner D1 PPU driver")
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>