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

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.

CC: <stable@vger.kernel.org>
Signed-off-by: Eugene Surovegin <surovegin@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>