shell: add devmem load command

This commit adds a devmem load command for shell that allows
users to easily load arbitrary data into the device memory.

Signed-off-by: Mateusz Sierszulski <msierszulski@internships.antmicro.com>
Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>