ARM: stm32: Fix livetree conversion on STM32MP15xx DHSOM

ARM: stm32: Fix livetree conversion on STM32MP15xx DHSOM

Unlike fdt_node_check_compatible() which returns 0 if node is compatible,
ofnode_device_is_compatible() return true which is non-zero if node is
compatible. The intention of the code is to exit from the function in
case the node is not compatible with "micrel,ks8851-mll". Add the missing
invert into the conditional to reinstate original behavior.

This exposes a follow up problem caused by conversion to DM based FMC2 EBI
driver, where the FMC2 EBI is not configured when accessed by this code.
Probe the KS8851 MAC, which also configures the FMC2 EBI as a dependency,
so that the KS8851 MAC CCR register can be accessed over the FMC2 EBI bus
and checked for EEPROM present bit.

Fixes: 5a605b7c8615 ("board: dhelectronics: stm32mp1: convert to livetree")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>