fix(context mgmt): remove explicit ICC_SRE_EL2 register read

ICC_SRE_EL2 has only 4 bits, while others are RES0. The library programs
all four of them already, so there is no need to read the previous
settings from the actual register.

This patch removes the explicit register read as a result.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: Iff0cb3b0d6fd85e5ae891068e440d855973a1c5e