ArmPlatformPkg/NorFlashDxe: eliminate void pointer arithmetic

ArmPlatformPkg/NorFlashDxe: eliminate void pointer arithmetic

While most compilers happily allow arithmetic on void pointer,
the RVCT compiler does not, and throws the following warning for
NorFlashDxe:

  ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashDxe.c(894,48) :
  error  #1254-D: arithmetic on pointer to void or function type

Since the expression in question involves a cast from UINTN to VOID*,
simply add some parentheses to eliminate this warning.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>