parisc: Delete redundant register definitions in <asm/assembly.h>

We define sp and ipsw in <asm/asmregs.h> using ".reg", and when using
current binutils (snapshot 2.40.50.20230611) the definitions in
<asm/assembly.h> using "=" conflict with those:

arch/parisc/include/asm/assembly.h: Assembler messages:
arch/parisc/include/asm/assembly.h:93: Error: symbol `sp' is already defined
arch/parisc/include/asm/assembly.h:95: Error: symbol `ipsw' is already defined

Delete the duplicate definitions in <asm/assembly.h>.

Also delete the definition of gp, which isn't used anywhere.

Signed-off-by: Ben Hutchings <benh@debian.org>
Cc: stable@vger.kernel.org # v6.0+
Signed-off-by: Helge Deller <deller@gmx.de>