arch: x86: cast to the same size composite expression

Essential type of RHS operand (64 bit) is wider than essential
type of composite expression in LHS operand (32 bit).
LHS entry_val is 32 bit, and RHS (phys+offset) is 64 bit.
Cast RHS composite expression to the (pentry_t) type.

Found as a coding guideline violation (MISRA R10.7) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>