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 (the chip might be in 4BA mode after reset). If we can't assure
the state of 4BA mode, we bail out to simplify the code (we'd have to
ensure that we don't run any instructions that can usually be switched
to 4BA mode otherwise).

Two new feature flags are introduced:

* 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 is dropped, it's completely implicit now.

Also, draw the with/without WREN distinction into the enter/exit
functions to reduce code redundancy.

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, 36 insertions(+), 49 deletions(-)