powerpc/asm: Define STACK_PT_REGS_OFFSET macro in asm-offsets.c

There are quite a few entries in asm-offests.c which look like:

  DEFINE(REG, STACK_FRAME_OVERHEAD+offsetof(struct pt_regs, reg));

So define a macro to do it once.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
[mpe: Rename to STACK_PT_REGS_OFFSET for excruciating explicitness]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>