memory: tegra20-emc: fix an OF node reference bug in tegra_emc_find_node_by_ram_code()

In the Linux kernel, the following vulnerability has been resolved:

memory: tegra20-emc: fix an OF node reference bug in tegra_emc_find_node_by_ram_code()

As of_find_node_by_name() release the reference of the argument device

node, tegra_emc_find_node_by_ram_code() releases some device nodes while

still in use, resulting in possible UAFs. According to the bindings and

the in-tree DTS files, the "emc-tables" node is always device's child

node with the property "nvidia,use-ram-code", and the "lpddr2" node is a

child of the "emc-tables" node. Thus utilize the

for_each_child_of_node() macro and of_get_child_by_name() instead of

of_find_node_by_name() to simplify the code.

This bug was found by an experimental verification tool that I am

developing.

[krzysztof: applied v1, adjust the commit msg to incorporate v2 parts]