phy: test: Implement sandbox PHY .set_mode and DM test

phy: test: Implement sandbox PHY .set_mode and DM test

Implement trivial extension to the sandbox PHY, which makes it pretend
to support selecting USB Host mode and nothing else. Any other mode is
rejected with -EINVAL. Any submode except for default submode 0 is
rejected with -EOPNOTSUPP . The implementation behaves in this trivial
way to permit easy unit testing using test which is also added in this
commit.

To run the test, use e.g. sandbox64_defconfig and run U-Boot as follows:
$ ./u-boot -Tc 'ut dm phy_setup'

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>