powerpc: Use LOAD_REG_IMMEDIATE only for constants on 64-bit

Using LOAD_REG_IMMEDIATE to get the address of kernel symbols
generates 5 instructions where LOAD_REG_ADDR can do it in one,
and will generate R_PPC64_ADDR16_* relocations in the output when
we get to making the kernel as a position-independent executable,
which we'd rather not have to handle.  This changes various bits
of assembly code to use LOAD_REG_ADDR when we need to get the
address of a symbol, or to use suitable position-independent code
for cases where we can't access the TOC for various reasons, or
if we're not running at the address we were linked at.

It also cleans up a few minor things; there's no reason to save and
restore SRR0/1 around RTAS calls, __mmu_off can get the return
address from LR more conveniently than the caller can supply it in
R4 (and we already assume elsewhere that EA == RA if the MMU is on
in early boot), and enable_64b_mode was using 5 instructions where
2 would do.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/include/asm/ppc_asm.h

@@ -268,7 +268,7 @@ n:
  *   Loads the value of the constant expression 'expr' into register 'rn'
  *   using immediate instructions only.  Use this when it's important not
  *   to reference other data (i.e. on ppc64 when the TOC pointer is not
- *   valid).
+ *   valid) and when 'expr' is a constant or absolute address.
  *
  * LOAD_REG_ADDR(rn, name)
  *   Loads the address of label 'name' into register 'rn'.  Use this when