ramips: specify "firmware" partition format

ramips: specify "firmware" partition format

Specify firmware partition format by compatible string.

formats in ramips:

- denx,uimage
- tplink,firmware
- seama

It's unlikely but the firmware splitting might not work any longer for
the following boards, due to a custom header:

- EX2700: two uImage headers
- BR-6478AC-V2: edimax-header
- 3G-6200N: edimax-header
- 3G-6200NL: edimax-header
- BR-6475ND: edimax-header
- TEW-638APB-V2: umedia-header
- RT-N56U: mkrtn56uimg

But it rather looks like the uImage splitter is fine with the extra
header.

The following dts are not touched, due to lack of a compatible string in
the matching firmware splitter submodule:

- CONFIG_MTD_SPLIT_JIMAGE_FW
    DWR-116-A1.dts
    DWR-118-A2.dts
    DWR-512-B.dts
    DWR-921-C1.dts
    LR-25G001.dts
- CONFIG_MTD_SPLIT_TRX_FW
    WCR-1166DS.dts
    WSR-1166.dts
- CONFIG_MTD_SPLIT_MINOR_FW
    RBM11G.dts
    RBM33G.dts
- CONFIG_MTD_SPLIT_LZMA_FW
    AR670W.dts
- CONFIG_MTD_SPLIT_WRGG_FW
    DAP-1522-A1.dts

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
(backported from d70ec3008d4cd0540a9f6c88fb7e786107f1679a)