quark_d2000: Update UART_IRQ_FLAGS for the board

Since we started to use UART_IRQ_FLAGS define them also for
quark_d2000 board. Note that we use IOAPIC_LEVEL since this is right
thing to do and the issue with lost interrupts is fixed in
_loapic_isr_vector_get().

Change-Id: Iaed0283bcc01290e420b42ff5d40ce9d99bfc677
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>