cmd: bcb: fix bcb struct alignment issue

Without this patch the bcb struct could be located at an odd address
which resulted in data not being copied to the buffer.

Here was the repro steps (from Mattijs):
=> mmc dev 1
=> bcb load 1 misc
=> bcb dump command
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=> part start mmc 1 misc misc_start
=> mmc read ${loadaddr} ${misc_start} 4
=> bcb load 1 misc
=> bcb dump command
00000000: 62 6f 6f 74 6f 6e 63 65 2d 62 6f 6f 74 6c 6f 61
00000010: 64 65 72 00 00 00 00 00 00 00 00 00 00 00 00 00

This behavior was observed on an Amlogic A311D (ARM64) platform with a
recent GCC toolchain (11.2.0) but is most likely affecting other
platforms.

To avoid issues the structure is aligned on DMA minimum alignment value
as it is passed directly to the read function.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on khadas vim3