nvmem: u-boot-env: add Broadcom format support

nvmem: u-boot-env: add Broadcom format support

Broadcom uses U-Boot for a lot of their bcmbca familiy chipsets. They
decided to store U-Boot environment data inside U-Boot partition and to
use a custom header (with "uEnv" magic and env data length).

Add support for Broadcom's specific binding and their custom format.

Ref: 6b0584c19d87 ("dt-bindings: nvmem: u-boot,env: add Broadcom's variant binding")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20221118063932.6418-9-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>