powerpc/prom_init: Add the ESM call to prom_init

Make the Enter-Secure-Mode (ESM) ultravisor call to switch the VM to secure
mode. Pass kernel base address and FDT address so that the Ultravisor is
able to verify the integrity of the VM using information from the ESM blob.

Add "svm=" command line option to turn on switching to secure mode.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
[ andmike: Generate an RTAS os-term hcall when the ESM ucall fails. ]
Signed-off-by: Michael Anderson <andmike@linux.ibm.com>
[ bauerman: Cleaned up the code a bit. ]
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190820021326.6884-5-bauerman@linux.ibm.com