[POWERPC] iSeries: Clean up lparmap mess

We need to have xLparMap in head_64.S so that it is at a fixed address
(because the linker will not resolve (address & 0xffffffff) for us).
But the assembler miscalculates the KERNEL_VSID() expressions.  So put
the confusing expressions into asm-offsets.c.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>