OMAP: EMIF: fix LP_MODE_PWR_DN value definition

OMAP: EMIF: fix LP_MODE_PWR_DN value definition

According to TRM the EMIF Power-down mode is identified
by value 0x4 (EMIF_PWR_MGMT_CTRL.REG_LP_MODE)

Change-Id: I4d9b9534dea27ab4619af5b6d78de8ab14f0fead
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>