video: fbdev: pvr2fb: add COMPILE_TEST support

Add COMPILE_TEST support to pvr2fb driver for better compile
testing coverage.

While at it:

- mark pvr2fb_interrupt() and pvr2fb_common_init() with
  __maybe_unused tag (to silence build warnings when
  !SH_DREAMCAST)

- convert mmio_base in struct pvr2fb_par to 'void __iomem *'
  from 'unsigned long' (needed to silence build warnings on
  ARM).

- split pvr2_get_param() on pvr2_get_param_name() and
  pvr2_get_param_val() (needed to silence build warnings on
  x86).

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>