mpc83xx: Don't allow W and G bits in IBATs

mpc83xx: Don't allow W and G bits in IBATs

The "Programming Environments Manual for 32-Bit Implementations of the
PowerPC™ Architecture" says "W and G bits are not defined for IBAT
registers. Attempting to write to these bits causes boundedly-undefined
results"

The "e300 Power Architecture™ Core Family Reference Manual" says the
same: "Neither the W or G bits of the IBAT registers should be set.
Attempting to write to these bits causes boundedly-undefined results."

Remove the possibility to set those bytes.

Fixes: 30915ab95d9 ("mpc83xx: Migrate BATS config to Kconfig")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Mario Six <mario.six@gdsys.cc>