ARM: sunxi_defconfig: enable CONFIG_REGULATOR

Commit 97a13e52

 ('net: phy: mdio-sun4i: don't select REGULATOR') removed
the select of REGULATOR, which means that it now has to be explicitly
enabled in the defconfig or things won't work very well.

In particular, this fixes a problem with SD/MMC not probing on my A31-based
board.

Cc: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>