tmimsp: support 4 KB sectors on AT25DF041A

The tmimsp platform family by default uses an Atmel AT25DF041A serial
FLASH part which supports 4 KB erase blocks.  Since the part's
communications protocol is a super-set of that supported by the STM25P
family, a private Stm25PSpiP that issues 0x20 4 KB erase block instead
of the default 0xd8 64 KB erase block command, and a companion Stm25p.h
that properly defines the sector size (aka erase block size) allows the
part to be treated as an array of 4 KB sectors.  This is a huge win for
most application uses of FLASH.