ARM: ixp4xx: fix compile-testing soc drivers

Randconfig builds on the ixp4xx ethernet driver showed that the qmgr and
npe drivers are not actually built even when compile testing is enabled:

ERROR: modpost: "qmgr_stat_empty" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "qmgr_enable_irq" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "qmgr_set_irq" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "__qmgr_request_queue" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "npe_send_recv_message" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "npe_recv_message" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "npe_load_firmware" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "npe_running" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "qmgr_disable_irq" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!
ERROR: modpost: "qmgr_stat_below_low_watermark" [drivers/net/ethernet/xscale/ixp4xx_eth.ko] undefined!

Fix it by always entering the drivers/soc/ixp4xx/ directory, and fix the
resulting compile test failures by removing the #include statements
that prevent building on most other platforms.

Fixes: 7a6c9dbb ("soc: ixp4xx: Protect IXP4xx SoC drivers by ARCH_IXP4XX || COMPILE_TEST")
Fixes: fcf2d897 ("ARM: ixp4xx: Move NPE and QMGR to drivers/soc")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210721211412.3537004-1-arnd@kernel.org

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>