tree/: Convert flashchip decode range func ptr to enum

Replace the `decode_range` function pointer in `struct flashchip` to an
enum value. The enum value can be used to find the corresponding
function pointer by passing it to `lookup_decode_range_func_ptr()`.

Removing function pointers like `decode_range` makes it possible to represent chip data in a declarative format that does not have to be
stored as C source code.

BUG=b:242479049
BRANCH=none
TEST=ninja && ninja test

Signed-off-by: Nikolai Artemiev <nartemiev@google.com>
Change-Id: If6d08d414d3d1ddadc95ca1d407fc87c23ab543d
---
M flashchips.c
M include/flash.h
M tests/chip_wp.c
M writeprotect.c
4 files changed, 72 insertions(+), 28 deletions(-)