powerpc/configs: Update skiroot defconfig

Disable new features from recent releases, and clean out some other
unused options:

  - Enable EXPERT, so we can disable some things
  - Disable non-powerpc BPF decoders
  - Disable TASKSTATS
  - Disable unused syscalls
  - Set more things to be modules
  - Turn off unused network vendors
  - PPC_OF_BOOT_TRAMPOLINE and FB_OF are unused on powernv
  - Drop unused Radeon and Matrox GPU drivers
  - IPV6 support landed in petitboot
  - Bringup related command line powersave=off dropped, switch to quiet

Set CONFIG_I2C_CHARDEV=y as the module is not loaded automatically, and
without this i2cget etc. will fail in the skiroot environment.

This defconfig gets us build coverage of KERNEL_XZ, which was broken in
the 4.19 merge window for powerpc.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>