MFC r314933-r314934

r314933:
[spigen] make spigen device ready to be compiled as a module

• Add flag to indicate that device is opened by userland
• Replace "always fail" detach method with proper detach implementation

r314934:
[spigen] Add spigen module

spigen provides userland API to SPI bus. Make it available as a loadable
module so people using official ARM images can enabled it on devices like
BBB or RPi without re-building kernel