Make Tock's mailbox match the TLUL Mailbox spec & addressess.

Match register names, make Tock only use the "SEC" endpoint of the
mailbox, tweak how interrupts work due to the wonky "WTIRQ" interrupt
that doesn't actually work for sleeping until a mailbox is drained
(greater-than vs less-than issue in the spec). SEC will now busy-wait
if the outbox is full, which should not usually be a problem.

Change-Id: Ibc4c220c39cfc8395aa7948efdf6022a85738d51