ARM: hawkboard: fix compilation of nand_spl

Fix build problem:

nand_spl/board/davinci/da8xxevm/hawkboard_nand_spl.c: In function 'board_init_f':
nand_spl/board/davinci/da8xxevm/hawkboard_nand_spl.c:132: warning: implicit declaration of function 'nand_boot'
nand_spl/board/davinci/da8xxevm/hawkboard_nand_spl.c:133: warning: 'noreturn' function does return

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Syed Mohammed Khasim <sm.khasim@gmail.com>
Cc: Sughosh Ganu <urwithsughosh@gmail.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>