ARM: mvebu: make CPU_PJ4B selection a per-SoC choice

Until now, the CPU_PJ4B Kconfig option was selected by
MACH_ARMADA_MVEBU, i.e for all Armada MVEBU SOCs. In preparation to
the introduction of Cortex-A9 based Armada MVEBU SOCs, this selection
is moved down to the Armada 370 and Armada XP specific options.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>