verdin-imx8m{m|p}: defconfig: update fastboot buffer size/address

verdin-imx8m{m|p}: defconfig: update fastboot buffer size/address

Remove FASTBOOT_BUF_SIZE from verdin-imx8mm_defconfig and
verdin-imx8mp_defconfig to use the default value of 0x7000000, aligning
with other Toradex boards.

Update FASTBOOT_BUF_ADDR to 0x44200000, consistent with the value in
previous downstream NXP U-Boot releases. This will make fastboot
applications continue working with upstream U-Boot by using the same
address.

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>