tegra: add enterrcm command

tegra: add enterrcm command

Tegra's boot ROM supports a mode whereby code may be downloaded and flash
programmed over a USB connection. On dev boards, this is typically entered
by holding down a "force recovery" button and resetting the CPU. However,
not all boards have such a button (one example is the Compulab Trimslice),
so a method to enter RCM from software is useful.

This change implements the command "enterrcm" to do this, and enables it
for all Tegra boards by default. Even on boards other than Trimslice,
controlling this over a UART may be useful, e.g. to allow simple remote
control without the need for mechanical button actuators, or hooking up
relays/... to the button.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>