configs: am64x_evm_*_defconfig: Increase offsets for eMMC raw boot

EMMC boot can fail due to the size of R5 SPL image growing beyond the
500KB of memory allocated in eMMC. Update offsets for eMMMC raw boot
to load each binary from the correct address in eMMC according to the
following eMMC layout:

            boot0/1 partition
    0x0+----------------------------------+
       |     tiboot3.bin (1 MB)           |
  0x800+----------------------------------+
       |       tispl.bin (2 MB)           |
0x1800+-----------------------------------+
       |       u-boot.img (4 MB)          |
0x3800+-----------------------------------+
       |      environment (128 KB)        |
0x3900+-----------------------------------+

Signed-off-by: Judith Mendez <jm@ti.com>