include: configs: Increase CONFIG_SYS_BOOTM_LEN to 16MB

The default value of CONFIG_SYS_BOOTM_LEN is 0x800000 i.e, 8MB which
causes board reset because of larger uImage size.

This was tested on rk3288 Amarula Vyasa and rk3288 Asus Tinker
boards.

Error log snippet:
   Booting using the fdt blob at 0x1f00000
   Loading Kernel Image ... Image too large: increase CONFIG_SYS_BOOTM_LEN
Must RESET board to recover
resetting ...

Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shyam Saini <shyam.saini@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>