input: simplify key_matrix_decode_fdt()

We know the exact property names that the code wants to process. Look
these up directly with fdt_get_property(), rather than iterating over
all properties within the node, and checking each property's name, in
a convoluted fashion, against the expected name.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
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Note: This depends on my previous patch "input: fix unaligned access
in key_matrix_decode_fdt()". While to two patches could be squashed
together, I'd prefer them to go in separately, since the former is a
bug-fix that makes the original code work again on ARMv7 at least, and
this patch is an unrelated refactoring.