am62a7: dts: Enable full 4GB LPDDR4

AM62A7-SK board has 4GB LPDDR4 Micron MT53E2G32D4DE-046 AUT:B part
but only 2GB was enabled early.

Enable full 4GB memory by updating the latter 2GB memory region
which gets mapped to 0x0880000000 i.e. DDR16SS0_SDRAM as referred in
Table 2-1. AM62A Common SoC Memory of AM62Ax TRM [1].

[1] : https://www.ti.com/lit/zip/spruj16
Logs: https://gist.github.com/devarsht/e85b6af89c01ddadb3a62f3e5f196af8



Signed-off-by: Devarsh Thakkar <devarsht@ti.com>