Use a TR of size 1 << IA64_ID_PAGE_SHIFT instead of 16M to avoid

Use a TR of size 1 << IA64_ID_PAGE_SHIFT instead of 16M to avoid
overlapping TR/TC entries (which results in a machine check). Note
that we don't look at the size of the memory descriptor, because
it doesn't guarantee non-overlap.

With this change, a UP kernel could boot on a Intel Tiger4 machine
with the following options:

options         LOG2_ID_PAGE_SIZE=26		# 64M
options         LOG2_PAGE_SIZE=14               # 16K

Approved by: marcel