Update git submodules

* Update src/sonic-linux-kernel from branch 'broadcom_sonic'
  to a73eea4085e47ec16b1d11d88645e4d315cc9a11
  - Merge 201904 branch to broadcom_sonic branch on Mon Jul  1 13:57:54 PDT 2019
    
    Change-Id: I0f38f03a2002b2bc1d6529a6147ef53a2c82e10b
    
  - [security] Upgrade kernel to 4.9.168-1+deb9u2 (sonic-net#86)
    
    * [security] Upgrade ekernel to 4.9.168-1+deb9u2
    
    * Backport dmi fix from linux-4.16 (sonic-net#85)
    
    This is just a cosmetic fix for platforms that doesn't provide any DMI
    information. Without this patch this raise an error kernel message which
    should actually just be a debug one. This patch does just that.
    
    * remove uart race condition patches as both of them are
    added to kernel 4.9.168
    
    2019-01-31 uart: Fix crash in uart_write and uart_put_char
    2018-09-09 uart: fix race between uart_put_char() and uart_shutdown()
    
    Signed-off-by: Guohan Lu <gulv@microsoft.com>
    
    * Add downloadable kernel packages
    
  - Backport dmi fix from linux-4.16 (sonic-net#85)
    
    This is just a cosmetic fix for platforms that doesn't provide any DMI
    information. Without this patch this raise an error kernel message which
    should actually just be a debug one. This patch does just that.