arm: armv8: Improve SPL data save and restore implementation

arm: armv8: Improve SPL data save and restore implementation

Introduce a new symbol in the beginning of .data section in
the common ARMv8 linker script and use that as a reference
for data save and restore.

Previously, the code would rely on calculating the start of
the .data section address via data size, however, we observed
that the data size does not really reflect the SPL mapped
addresses.

In our case, the binman_sym section size was not included in
the data size, which will result in a wrong address for the
.data start section, which prevents us from properly saving
and restoring SPL data.

This approach skips the calculation for the starting address
of the .data section, and instead just defines the beginning
address of the .data section and calling the symbol as needed,
in which we think as a simpler and much more robust method.

Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Signed-off-by: Tien Fong Chee <tien.fong.chee@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>