cros_ec: Support keyboard scanning with EC_CMD_GET_NEXT_EVENT

cros_ec: Support keyboard scanning with EC_CMD_GET_NEXT_EVENT

The cros_ec_keyb driver currently uses EC_CMD_MKBP_STATE to scan the
keyboard, but this host command was superseded by EC_CMD_GET_NEXT_EVENT
and unavailable on more recent devices (including gru-kevin), as it was
removed in cros-ec commit 87a071941b89 ("mkbp: Add support for buttons
and switches.") dated 2016-07-06.

The EC_CMD_GET_NEXT_EVENT has been available since cros-ec commit
d1ed75815efe ("MKBP event signalling implementation") dated 2014-10-20,
but it looks like it isn't included in firmware-* branches for at least
link, nyan-big, samus, snow, spring, panther and peach-pit which have
defconfigs in U-Boot. So this patch falls back to the old method if the
EC doesn't recognize the newer command.

The implementation is mostly adapted from Depthcharge commit
f88af26b44fc ("cros_ec: Change keyboard scanning method.").

On a gru-kevin, the current driver before this patch fails to read the
pressed keys with:

    out: cmd=0x60: 03 9d 60 00 00 00 00 00
    in-header: 03 fc 01 00 00 00 00 00
    in-data:
    ec_command_inptr: len=-1, din=0000000000000000
    check_for_keys: keyboard scan failed

However the keyboard works fine with the newer command:

    out: cmd=0x67: 03 96 67 00 00 00 00 00
    in-header: 03 ef 00 00 0e 00 00 00
    in-data: 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    ec_command_inptr: len=14, din=00000000f412df30
    key_matrix_decode: num_keys = 0
      0 valid keycodes found
    out: cmd=0x67: 03 96 67 00 00 00 00 00
    in-header: 03 df 00 00 0e 00 00 00
    in-data: 00 00 00 00 00 00 00 00 00 00 00 00 10 00
    ec_command_inptr: len=14, din=00000000f412df30
    key_matrix_decode: num_keys = 1
      valid=1, row=4, col=11
        keycode=28
      1 valid keycodes found
     {0d}

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>