[C25][DTS] Add notest about PCIEe reset signal implementation

On SECO Q7 Dev Kit 2.0 carrier board, a PCIE_RST# signal on Q7 connector is
routed to multiple PCIe slots simultaneously. As a result, in the existing
device tree, it was set as a common regulator for the whole PCIe subsystem:
pcie_rst_cmd: pcie_rstcmd.
Herewith this pin is not under control of any device driver.

Added comments to the device tree to inform users about this and how to
change this behaviour if different hardware design will be used.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>