power: Convert CONFIG_PMIC_AS3722 to Kconfig

This converts the following to Kconfig:
   CONFIG_PMIC_AS3722

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/nyan-big_defconfig b/configs/nyan-big_defconfig
index 63f1a6f..b132eb1 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_DM_PMIC=y
+CONFIG_PMIC_AS3722=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_PWM_TEGRA=y