imx6: drop duplicated bss memset and board_init_r() call

imx6: drop duplicated bss memset and board_init_r() call

bss section is cleared in crt0.S. board_init_r() is also
entered from crt0 code.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>