[IA64] Make PCDP work again.

Mark's patch added "attribute((packed))" for pcdp_uart, without
accounting for the fact that the structure definition _relied_ on
implicit padding by 6 bytes.  Fix is to make the padding explicit.

Signed-off-by: David Mosberger-Tang <David.Mosberger@acm.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>