refactor(partition): get GPT header location from MBR

GPT header is located in first LBA after MBR entry and mbr header has
details of beginning of first entry, so use mbr header entry first_lba
data to locate GPT header rather than GPT_HEADER_OFFSET.

GPT header size is available in gpt_header, so use that
rather than using DEFAULT_GPT_HEADER_SIZE.

The location of GPT entries is available once we parse gpt_header
and is available as partitiona_lba use that to load gpt_entries rather
than GPT_ENTRY_OFFSET.

Change-Id: I3c11f8cc9d4b0b1778a37fe342fb845ea4a4eff1
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>