ARM: convert arch_fixup_memory_node to a generic FDT fixup function

Some architecture needs extra device tree setup. Instead of adding
yet another hook, convert arch_fixup_memory_node to be a generic
FDT fixup function.

[maz: collapsed 3 patches into one, rewrote commit message]

Signed-off-by: Ma Haijun <mahaijuns@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>