arm: Typo s/PCI_IXP4XX_LEGACY/IXP4XX_PCI_LEGACY/

Kconfig symbol PCI_IXP4XX_LEGACY does not exist, but IXP4XX_PCI_LEGACY
does.

Fixes: d5d9f7ac

 ("ARM/ixp4xx: Make NEED_MACH_IO_H optional")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/82ce37c617293521f095a945a255456b9512769c.1626255077.git.geert+renesas@glider.be

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