dmaengine: stm32-mdma: remove GISR1 register

GISR1 was described in a not up-to-date documentation when the stm32-mdma
driver has been developed. This register has not been added in reference
manual of STM32 SoC with MDMA, which have only 32 MDMA channels.
So remove it from stm32-mdma driver.

Fixes: a4ffb13c

 ("dmaengine: Add STM32 MDMA driver")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20220504155322.121431-2-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>