arm64: defconfig: update RK8XX MFD config

Update the defconfig for the new RK8XX MFD config name,
which got split to add SPI support.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: c20e8c5b ("mfd: rk808: Split into core and i2c")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>