taurus: board updates

taurus changes:
- rename at91_spl_board_init to spl_board_init
  fixes problems with recovery button and nand erase sector 0
- adapt CONFIG_SPL_MAX_SIZE and CONFIG_SPL_BSS_MAX_SIZE
- add CONFIG_AT91_HW_WDT_TIMEOUT 15
- CONFIG_SF_DEFAULT_MODE SPI_MODE_3 not mode 0

Signed-off-by: Heiko Schocher <hs@denx.de>