arm64: defconfig: updates for 4.5

Based on requests, update our defconfig so that:

  - We don't build any modules
  - PL031 is enabled (RTC emulated by qemu)
  - Xen guest support is enabled
  - The Uniphier built-in I2C controller is enabled
  - PCI host controller drivers for the various arm64 SoCs are enabled
  - Device passthrough works on Seattle using SMMU and VFIO
  - The Hisilicon IRQ controller (mbigen) is enabled

Signed-off-by: Will Deacon <will.deacon@arm.com>