net: ipa: limit local processing context address

Not all of the bits of the LOCAL_PKT_PROC_CNTXT register are valid.
Until IPA v4.5, there are 17 valid bits (though the bottom three
must be zero).  Starting with IPA v4.5, 18 bits are valid.

Introduce proc_cntxt_base_addr_encoded() to encode the base address
for use in the register using only the valid bits.

Shorten the name of the register (omit "_BASE") to avoid the need to
wrap the line in the one place it's used.

Signed-off-by: Alex Elder <elder@linaro.org>