net: ipa: determine route table size from memory region

Currently we assume that any routing table contains a fixed number
of entries.  The number of entries in a routing table can actually
vary, depending only on the size of the IPA-local memory region used
to hold the table.

Stop assuming that a routing table has exactly 15 entries.  Instead,
determine the number of entries in a routing table by dividing its
memory region size by the size of an entry.

The number of entries is computed early, when ipa_table_mem_valid()
is called by ipa_table_init().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>