irqchip/imx-mu-msi: Fix wrong register offset for 8ulp

Offset 0x124 should be for IMX_MU_TSR, not IMX_MU_GSR.

Fixes: 70afdab9 ("irqchip: Add IMX MU MSI controller driver")
Reported-by: Colin King <colin.i.king@gmail.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[maz: updated commit message, tags]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221004202414.216577-1-Frank.Li@nxp.com