powerpc/hvsi: Increase handshake timeout from 200ms to 400ms.

powerpc/hvsi: Increase handshake timeout from 200ms to 400ms.

commit d220980b701d838560a70de691b53be007e99e78 upstream.

This solves a problem observed in kexec'ed kernel where 200ms timeout is
too short and bootconsole fails to initialize. Console did eventually
become workable but much later into the boot process.

Observed timeout was around 260ms, but I decided to make it a little bigger
for more reliability.

This has been tested on Power7 machine with Petitboot as a primary
bootloader and PowerNV firmware.

Signed-off-by: Eugene Surovegin <surovegin@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>