of: of_reserved_mem: Increase limit on number of reserved regions

Certain SoCs need to support a large amount of reserved memory
regions. For example, Qualcomm's SM8150 SoC requires that 20
regions of memory be reserved for a variety of reasons (e.g.
loading a peripheral subsystem's firmware image into a
particular space).

When adding more reserved memory regions to cater to different
usecases, the remaining number of reserved memory regions--12
to be exact--becomes too small. Thus, double the existing
limit of reserved memory regions.

Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>