fix(nxp/common/setup): fix total dram size checking

total_dram_size should be signed value because it is equal to return
value of init_ddr(), so if it is lower or equal zero, report
error as DDR is not initialized correctly.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>
Change-Id: Idbc40da103f60f10cb18c5306e97b764c1a9d372