hw/loongarch/virt: rename PCH_PIC_IRQ_OFFSET with VIRT_GSI_BASE

hw/loongarch/virt: rename PCH_PIC_IRQ_OFFSET with VIRT_GSI_BASE

In theory gsi base can start from 0 on loongarch virt machine,
however gsi base is hard-coded in linux kernel loongarch system,
else system fails to boot.

This patch renames macro PCH_PIC_IRQ_OFFSET with VIRT_GSI_BASE,
keeps value unchanged. GSI base is common concept in acpi spec
and easy to understand.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20221228030719.991878-1-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>