sh: add SMP support for J2

Support is hooked up via a cpu start method specified in the device
tree, and also depends on DT nodes that describe the interfaces for
performing IPI and identifying which cpu execution is taking place on.
The currently used method is a form of spin table, where secondary
cpus are unblocked by writing to a special address.

Signed-off-by: Rich Felker <dalias@libc.org>