tests: drivers: adc_dma: stm32h7: use auto generated linker section

Change this test to use the automatically generated linker section
to place the buffer in SRAM4, instead of using the manually created
region added in 088d38f. This is in preperation of removing the
manually created section.

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>