power: reset: use hard resets by default for all restart commands

Present, hard resets are used only when rebooting for "recovery",
"rtc", or "bootload" reboot commands or when the reboot command
is an empty string. Perform hard resets for invalid reboot commands
also, to avoid accidental warm resets if an invalid reboot command
is mistakenly used.

Continue to use warm resets for entry into download mode. This is
required for collection of ramdumps.

Change-Id: I71f657e9c8c20abcbbda86d789c843060c8ffce7
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>