acpigen: Add some new helper functions

acpigen: Add some new helper functions

These build on existing functions but use different object types
in order to provide functions for upcoming changes:

acpigen_write_return_op(): Return an operator.
acpigen_write_if_lequal_op_op(): Check if 2 operands are equal.
acpigen_get_package_op_element(): Read an element from a package
                                  into an operator.

This one just provides the missing helper, the other 3 already exist:
acpigen_get_tx_gpio: Read TX gpio state.

Signed-off-by: Duncan Laurie <dlaurie@google.com>
Change-Id: I1141fd132d6f09cf482f74e95308947cba2c5846
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41985
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>