x86: Reduce mrccache record alignment size

At present the records are 4KB in size. This is unnecessarily large when
the SPI-flash erase size is 256 bytes. Reduce it so it will be more
efficient with Apollo Lake's 24-byte variable-data record.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>