arm64: configs: add options useful for Armada 7K/8K support

This commit updates the ARM64 defconfig to include additional options
useful to support the Armada 7K/8K platforms:

 - the SPI controller driver, spi-orion
 - the support for SPI flashes

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>