imx9: scmi: Fix SPL trampoline buffer for 1GB DDR

After supporting get DRAM size from SM, the trampoline buffer address
still depends on PHYS_SDRAM_SIZE. If the real DDR size is less than
PHYS_SDRAM_SIZE, the trampoline buffer address is invalid and SPL will
crash. So use board_phys_sdram_size to get real DDR size to calculate
correct address.

Fixes: e1cc7117b630 ("imx9: scmi: Get DDR size through SM SCMI API")
Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>