verdin-am62: fix FASTBOOT_BUF_ADDR and FASTBOOT_BUF_SIZE

verdin-am62: fix FASTBOOT_BUF_ADDR and FASTBOOT_BUF_SIZE

The current value of CONFIG_FASTBOOT_BUF_ADDR is incorrect for hardware
with 512MB and 1GB of RAM. The RAM address ranges for these devices are:

- 512MB:
  - Start: 0x80000000
  - End:   0xA0000000
- 1GB:
  - Start: 0x80000000
  - End:   0xC0000000

The current buffer address makes fastboot downloads work only on
hardware with 2GB of RAM.

To fix this issue:
1. Lower CONFIG_FASTBOOT_BUF_ADDR to 0x88200000. This makes the buffer
   to be equal ot CONFIG_SYS_LOAD_ADDR, which is a safe address to
   download files using fastboot.

2. Reduce the size of the download buffer to 128MB, avoiding overlaps
   with RAMDISK_ADDR_R. This is a good enough value for individual
   downloads.

These changes make fastboot downloads work with 512MB and 1GB devices.

Fixes: defe30a78b76 ("verdin-am62: add DFU, USB and UUU fastboot support")
Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com>