openpower: pdbg: Bump version to v3.6

gdbserver improvements:

 - Power9 and Power10 support
 - Multi thread debugging support
 - Updates to parser

libpdbg changes:

 - Remove attn API, it was unused and is now part of the gdbserver
 - Change behaviour of sbe_get_state to suit phal consumer
 - Fix thread_stop
 - Remove printing from thread_regs functions

Signed-off-by: Joel Stanley <joel@jms.id.au>
Change-Id: I5101cbd6df3c92fc11ea323f8ae6db4d18ddb5cf