ppc: Rework POWER7 & POWER8 exception model

ppc: Rework POWER7 & POWER8 exception model

From: Benjamin Herrenschmidt <[email protected]>

This patch fixes the current AIL implementation for POWER8. The
interrupt vector address can be calculated directly from LPCR when the
exception is handled. The excp_prefix update becomes useless and we
can cleanup the H_SET_MODE hcall.

Signed-off-by: Benjamin Herrenschmidt <[email protected]>
[clg: Removed LPES0/1 handling for HV vs. !HV
      Fixed LPCR_ILE case for POWERPC_EXCP_POWER8 ]
Signed-off-by: Cédric Le Goater <[email protected]>
[dwg: This was written as a cleanup, but it also fixes a real bug
      where setting an alternative interrupt location would not be
      correctly migrated]
Signed-off-by: David Gibson <[email protected]>