phy: Remove ARCH_KIRKWOOD dependency

mach-kirkwood has been removed, now that kirkwood lives in mach-mvebu.
Depend on MACH_KIRKWOOD, which will be set when these SoCs are built
as part of mach-mvebv.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>