fix(tc): change the FIP offset to 8 KiB boundary

* This change overrides the default PLAT_ARM_FIP_OFFSET_IN_GPT

* This aligns the FIP base in GPT image to the RSS ATU page size
  boundary (8 KiB). RSS XIP feature requires the FIP to be aligned to
  the page size boundary. TC platform will require the XIP feature.

* The aligned FIP_A is starting at sector 48. Hence the offset will be
  48*512 = 0x6000.

Signed-off-by: Tintu Thomas <tintu.thomas@arm.com>
Change-Id: I8135ecd4168231847c80151c33ef8353a1586b9a