powerpc/pkeys: Fix boot failures with Nemo board (A-EON AmigaOne X1000)

On p6 and before we should avoid updating UAMOR SPRN. This resulted
in boot failure on Nemo board.

Fixes: 269e829f ("powerpc/book3s64/pkey: Disable pkey on POWER6 and before")
Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200810102623.685083-1-aneesh.kumar@linux.ibm.com