kernel: re-add missing symbols

kernel: re-add missing symbols

a recent cleanup commit also removed our own custom symbols
added by various in-tree drivers.

re-add these symbols as the compiler will complain about
them otherwise on specific targets during kernel build:

MikroTik RB91x NAND driver support (MTD_NAND_RB91X) [N/m/y/?] (NEW) make[8]: *** [scripts/kconfig/Makefile:77: syncconfig] Error 1

Fixes: 2d00ed7caa ("kernel: trim outdated kernel config symbols")
Signed-off-by: Koen Vandeputte <koen.vandeputte@citymesh.com>