am33xx: Stop using PHYS_DRAM_1 define

We defined PHYS_DRAM_1 to 0x80000000 (start of DRAM) and then used this
for CONFIG_SYS_SDRAM_BASE.  But then we kept on referencing PHYS_DRAM_1
in other places.  Change to directly setting CONFIG_SYS_DRAM_BASE and
then using that name in code.

Signed-off-by: Tom Rini <trini@ti.com>