Moving AMBA/GRLIB drivers to bsps/shared

[Jiri Gaisler]

Hello Sebastian,

I have started to copy the basic AMBA p&p routines and the drivers for
irqmp and gptimer to bsps/shared/amba. The routines are somewhat
entangled with LEON3-specific code, so I have cleaned them up and made
them cpu independent. I don't intend to touch the original code under
bsps/sparc, so the leon3 bsps should not be affected. To avoid building
the drivers for all architectures, I have added an --enable-grlib option
to configure. I am now working on bringing up my RISCV/GRLIB bsp. Once
that is working, I will submit a patch for review.

Jiri.