ARM: dts: vf610-bk4: Fix qspi node description

Before this change the device tree description of qspi node for
second memory on BK4 board was wrong (applicable to old, removed
fsl-quadspi.c driver).

As a result this memory was not recognized correctly when used
with the new spi-fsl-qspi.c driver.

From the dt-bindings:

"Required SPI slave node properties:
  - reg: There are two buses (A and B) with two chip selects each.
This encodes to which bus and CS the flash is connected:
<0>: Bus A, CS 0
<1>: Bus A, CS 1
<2>: Bus B, CS 0
<3>: Bus B, CS 1"

According to above with new driver the second SPI-NOR memory shall
have reg=<2> as it is connected to Bus B, CS 0.

Fixes: a67d2c52 ("ARM: dts: Add support for Liebherr's BK4 device (vf610 based)")
Suggested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>