spi25: Revise decision when to enter/exit 4BA mode

Instead of arbitrarily deciding whether to enter 4BA mode in the flash
chip's declaration, advertise that entering 4BA mode is supported and
only enter it if the SPI master supports 4-byte addresses. If not, exit
4BA mode (as we don't know if the chip is in its reset state). Two new
feature flags are introduced for that:

* FEATURE_4BA_ENTER:
  Can enter/exit 4BA mode with instructions 0xb7/0xe9 w/o WREN.
* FEATURE_4BA_ENTER_WREN
  Can enter/exit 4BA mode with instructions 0xb7/0xe9 after WREN.

FEATURE_4BA_SUPPORT was dropped, it's completely implicit now.

Change-Id: I877fe817f801fc54bd0ee2ce4e3ead324cbb3673
Signed-off-by: Nico Huber <nico.h@gmx.de>
---
M chipdrivers.h
M flash.h
M flashchips.c
M flashrom.c
M spi25.c
5 files changed, 37 insertions(+), 50 deletions(-)