powerpc: Add current_stack_pointer as a register global

current_stack_frame() doesn't return the stack pointer, but the
caller's stack frame. See commit bfe9a2cf ("powerpc: Reimplement
__get_SP() as a function not a define") and commit
acf620ec ("powerpc: Rename __get_SP() to current_stack_pointer()")
for details.

In some cases this is overkill or incorrect, as it doesn't return the
current value of r1.

So add a current_stack_pointer register global to get the value of r1
directly.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of other patch, tweak change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200220115141.2707-2-mpe@ellerman.id.au

arch/powerpc/include/asm/reg.h

@@ -1450,6 +1450,8 @@ static inline void mtsrin(u32 val, u32 idx)
 
 extern unsigned long current_stack_frame(void);
 
+register unsigned long current_stack_pointer asm("r1");
+
 extern unsigned long scom970_read(unsigned int address);
 extern void scom970_write(unsigned int address, unsigned long value);