parisc: Remove lock code to serialize TLB operations in pacache.S

TLB operations only need to be serialized on machines with the Merced
(Stretch) bus. The only machines in this category are L and N class, and
they require a 64-bit PA 2.0 kernel. On these machines, we use local TLB
purges in the tmpalias routines.
We don't need to serialize TLB purges on all other machines. Thus, the
lock/unlock code can be removed when CONFIG_PA20 is not defined.
Further, when CONFIG_PA20 is not defined, alternative patching converts
the TLB purges to local purges when PA 2.0 hardware has been detected.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Tested-By: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>