staging: dwc2: add driver parameter to set AHB config register value

The dwc2 driver sets the value of the DWC2 GAHBCFG register to 0x6,
which is GAHBCFG_HBSTLEN_INCR4. But different platforms may require
different values. In particular, the Broadcom 2835 SOC used in the
Raspberry Pi needs a value of 0x10, otherwise the DWC2 controller
stops working after a short period of heavy USB traffic.

So this patch adds another driver parameter named 'ahbcfg'. The
default value is 0x6. Any platform needing a different value should
add a DT attribute to set it.

This patch also removes the 'ahb_single' driver parameter, since
that bit can now be set using 'ahbcfg'.

This patch does not add DT support to platform.c, I will leave that
to whoever owns the first platform that needs a non-default value.
(Stephen?)

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>