ptp: ixp46x: move adjacent to ethernet driver

The ixp46x ptp driver has a somewhat unusual setup, where the ptp
driver and the ethernet driver are in different directories but
access the same registers that are defined a platform specific
header file.

Moving everything into drivers/net/ makes it look more like most
other ptp drivers and allows compile-testing this driver on
other targets.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>