arm64: defconfig: Enable MAX8973 regulator

The Tegra210 Smaug board uses MAX77621 for both CPU & GPU rail. Note
that max8973 and max77621 share the same driver. So enable this driver
for the PMIC.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>