board: grouper: convert ASUS Google Nexus 7 (2012) to use DM PMIC

Since required drivers were merged, we can safely clean up the
board and switch to DM based driver with device tree support.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
diff --git a/configs/grouper_common_defconfig b/configs/grouper_common_defconfig
index 258d7b1..616540f 100644
--- a/configs/grouper_common_defconfig
+++ b/configs/grouper_common_defconfig
@@ -43,6 +43,7 @@
 CONFIG_CMD_UMS_ABORT_KEYED=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_PAUSE=y
+CONFIG_CMD_REGULATOR=y
 CONFIG_CMD_EXT4_WRITE=y
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set