brcmfmac: Add support for host platform NVRAM loading.

Host platforms such as routers supported by OpenWRT can
support NVRAM reading directly from internal NVRAM store.
With this patch the nvram load routines will fall back to
this method when there is no nvram file and support is
available in the kernel.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>