xen/arm: introduce a driver for the ARM HDLCD controller

xen/arm: introduce a driver for the ARM HDLCD controller

Read the screen resolution setting from device tree, find the
corresponding modeline in a small table of standard video modes, set the
hardware accordingly.

Use vexpress_syscfg to configure the pixel clock.

Use the generic framebuffer functions to print on the screen.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>