brcm: Add a link to enable khadas VIM2's WiFi

According to kernel message on khadas VIM2 board equipped with BCM4356:

brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac4356-sdio for chip BCM4356/2
usbcore: registered new interface driver brcmfmac
brcmfmac mmc0:0001:1: Direct firmware load for brcm/brcmfmac4356-sdio.khadas,vim2.txt failed with error -2
brcmfmac mmc0:0001:1: Direct firmware load for brcm/brcmfmac4356-sdio.txt failed with error -2

System needs the NVRAM file "brcmfmac4356-sdio.khadas,vim2.txt" to
enable the WiFi chip BCM4356. Found it can share with the same file
"brcmfmac4356-sdio.vamrs,rock960.txt" with a soft link as
"brcmfmac4356-sdio.khadas,vim2.txt". Both 2.4GHz and 5GHz WiFi are
enabled with the linked config.

This patch adds the link to brcmfmac4356-sdio.vamrs,rock960.txt for it.

Signed-off-by: Jian-Hong Pan <jhp@endlessos.org>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>