of: reserved-memory: remove duplicated call to of_get_flat_dt_prop() for no-map node

Just use nomap instead of the second call to of_get_flat_dt_prop(). And
change nomap as a bool type due to != NULL operator. Also, correct comment
about node of 'align' -> 'alignment'.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20200730092353.15644-1-zbestahu@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>