MA-17353-1 Add system board reset command for imx8q devices

MA-17353-1 Add system board reset command for imx8q devices

Default 'reset' command in bootloader will only trigger partition reboot
for A core, which will not reload everything (like SPL, SCFW, M4, etc.).
Sometimes, we want to trigger a board reboot without pressing the reset
button, it can be very helpful in some cases like remote debugging...

This commit adds command 'reboot' to trigger system board reboot via the
psci interface provided by ATF, it's enabled on i.MX 8 Quad platforms only.

Test: system board reboot on imx8qm/imx8qxp.

Change-Id: I27f2291806b6959d46fa6f55fe186041d141d2f4
Signed-off-by: Ji Luo <ji.luo@nxp.com>
(cherry picked from commit f8a127354ba4f77ba5b1af87f692133bb704cfe0)
(cherry picked from commit 646b99c6bcba96c97d3517a2c21d54908a8c4ed5)