ARM: dts: stm32: fix SAI sub nodes register range

The STM32 SAI subblocks registers offsets are in the range
0x0004 (SAIx_CR1) to 0x0020 (SAIx_DR).
The corresponding range length is 0x20 instead of 0x1c.
Change reg property accordingly.

Fixes: 5afd65c3 ("ARM: dts: stm32: add sai support on stm32mp157c")

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>