Define the fields for the ENDP_INIT_CTRL, ENDP_INIT_CFG, ENDP_INIT_NAT,
ENDP_INIT_HDR, and ENDP_INIT_HDR_EXT IPA registers for all supported
IPA versions.

Create enumerated types to identify fields for these IPA registers.
Use IPA_REG_STRIDE_FIELDS() to specify the field mask values defined
for these registers, for each supported version of IPA.

Move ipa_header_size_encoded() and ipa_metadata_offset_encoded() out
of "ipa_reg.h" and into "ipa_endpoint.c".  Change them so they take
an additional ipa_reg structure argument, and use ipa_reg_encode()
to encode the parts of the header size and offset prior to writing
to the register.  Change their names to be verbs rather than nouns.

Use ipa_reg_encode(), ipa_reg_bit, and ipa_reg_field_max() to
manipulate values to be written to these registers, remove the
definition of the no-longer-used *_FMASK symbols.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>