LoongArch: Use ACPI_GENERIC_GSI for gsi handling

For LoongArch, generic gsi code(driver/acpi/irq.c) can be
reused after following patchs:

APCI: irq: Add support for multiple GSI domains
ACPI: irq: Allow acpi_gsi_to_irq() to have an arch-specific fallback

So, config ACPI_GENERIC_GSI for LoongArch with removing the gsi code
in arch directory.

Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1658314292-35346-6-git-send-email-lvjianmin@loongson.cn