platform: cznic: CZNIC_PLATFORMS should depend on ARCH_MVEBU

CZ.NIC's Turris devices are based on Marvell EBU SoCs.  Hence add a
dependency on ARCH_MVEBU, to prevent asking the user about these drivers
when configuring a kernel that cannot run on an affected CZ.NIC Turris
system.

Fixes: 992f1a3d ("platform: cznic: Add preliminary support for Turris Omnia MCU")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>