of: Make of_get_ddrtype module friendly

The APIs fdt_path_offset() and fdt_getprop_w() are not exported.
As a result, drivers that are compiled as modules cannot directly
use them. Hence, switch to of_find_node_by_path() and
of_property_read_u32() to achive the same functionality while also
being module friendly.

Change-Id: I90b299c60b60a006d0932442549c5d5a20cf2ea0
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>