rp2/modmachine: Do not use deprecated XOSC_MHZ and XOSC_KHZ.

XOSC_MHZ and XOSC_KHZ may not be defined if we use a custom XIN clock
by defining PLL_SYS_REFDIV etc. calculated by vcocalc.py.

Signed-off-by: Christian Lang <lang.chr86@gmail.com>