tcgbios: Fix endianess of NumberOfPartitions

Write the 64bit integer 'NumberOfPartitions' in little endian format into
the uefi_gpt_data so that they end up in little endian format in the
measurement log.

Fixes: 16a1867425ee ("tcgbios: Measure the GPT table")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>