zephyr-kernel-src: add 2.6.0 release

Zephyr 2.6.0 release notes:
  https://github.com/zephyrproject-rtos/zephyr/releases/tag/zephyr-v2.6.0



Update the following submodules to most recent revisions:
- hal_nordic:
    nrfx_dppi: Fix variable used in logging function
- hal_stm32:
    dts/st: use a pull-down for SPI SCK pin
- openamp:
    lib: update open-amp lib to release v2021.04.0
- libmetal:
    lib: update libmetal to release v2021.04.0 + VERSION path fix

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>
Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>