PowerPC: Rename __kernel_vdso_get_tbfreq to __kernel_get_tbfreq.

PowerPC: Rename __kernel_vdso_get_tbfreq to __kernel_get_tbfreq.

In order for the __kernel_get_tbfreq vDSO call to work the
INTERNAL_VSYSCALL_NCS macro needed to be updated to prevent it from
assuming an integer return type (since the timebase frequency is a 64-bit
value) by specifying the type of the return type as a macro parameter.  The
macro then specifically declares the return value as a 'register' (or
implied pair) of the denoted type.  The compiler is then informed that this
register (or implied pair) is to be used for the return value.