powerpc/configs: Remove old symbols from defconfigs

Update defconfigs to remove old symbols and comments referencing old
symbols.

Dropped:

* AVERAGE
* INET_LRO
* EXT3_DEFAULTS_TO_ORDERED
* EXT3_FS_XATTR
* I2O
* INFINIBAND_AMSO1100
* INFINIBAND_EHCA
* IP1000

Replaced:

* BLK_DEV_XIP -> BLK_DEV_RAM_DAX
* CLK_PPC_CORENET -> CLK_QORIQ
* EXT2_FS_XIP -> FS_DAX
* EXT3_FS* -> EXT4_FS*

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>