arm64: kdump: reserve memory for crash dump kernel

arm64: kdump: reserve memory for crash dump kernel

"crashkernel=" kernel parameter specifies the size (and optionally
the start address) of the system ram to be used by crash dump kernel.
reserve_crashkernel() will allocate and reserve that memory at boot time
of primary kernel.

The memory range will be exposed to userspace as a resource named
"Crash kernel" in /proc/iomem.

Signed-off-by: AKASHI Takahiro <[email protected]>
Signed-off-by: Mark Salter <[email protected]>
Signed-off-by: Pratyush Anand <[email protected]>
Reviewed-by: James Morse <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>