hw/loongarch: Fix length for lowram in ACPI SRAT

hw/loongarch: Fix length for lowram in ACPI SRAT

The size of lowram should be "gap" instead of the whole node.

This is failing kernel's sanity check:

[    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00000000-0xffffffff]
[    0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x80000000-0x16fffffff]
[    0.000000] ACPI: SRAT: Node 1 PXM 1 [mem 0x170000000-0x26fffffff]
[    0.000000] Warning: node 0 [mem 0x00000000-0xffffffff] overlaps with itself [mem 0x80000000-0x16fffffff]

Fixes: fc100011f38d ("hw/loongarch: Refine acpi srat table for numa memory")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>