arm64: Fix SCTLR_EL1 initialisation

We initialise the SCTLR_EL1 value by read-modify-writeback
of the desired bits, leaving the other bits (including reserved
bits(RESx)) untouched. However, sometimes the boot monitor could
leave garbage values in the RESx bits which could have different
implications. This patch makes sure that all the bits, including
the RESx bits, are set to the proper state, except for the
'endianness' control bits, EE(25) & E0E(24)- which are set early
in the el2_setup.

Updated the state of the Bit[6] in the comment to RES0 in the
comment.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>