cmd: boot: Update reset usage message

The commit 573a3811 ("sysreset: psci: support system reset in a generic
way with PSCI") has added support for warm reset via PSCI but this hasn't
been reflected in usage message and user has to look at the code how to run
it. That's why update usage text to make this clear.

Here is full help with updated usage:
ZynqMP> help reset
reset - Perform RESET of the CPU

Usage:
reset - cold boot without level specifier
reset -w - warm reset if implemented

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>