of: fdt: Add memory for devices by DT property "linux,usable-memory-range"

When reserving crashkernel in high memory, some low memory is reserved
for crash dump kernel devices and never mapped by the first kernel.
This memory range is advertised to crash dump kernel via DT property
under /chosen,
        linux,usable-memory-range = <BASE1 SIZE1 [BASE2 SIZE2]>

We reused the DT property linux,usable-memory-range and made the low
memory region as the second range "BASE2 SIZE2", which keeps compatibility
with existing user-space and older kdump kernels.

Crash dump kernel reads this property at boot time and call memblock_add()
to add the low memory region after memblock_cap_memory_range() has been
called.

Signed-off-by: Chen Zhou <[email protected]>
Co-developed-by: Zhen Lei <[email protected]>
Signed-off-by: Zhen Lei <[email protected]>
Reviewed-by: Rob Herring <[email protected]>
Tested-by: Dave Kleikamp <[email protected]>
Acked-by: Baoquan He <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

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