powerpc: define helpers to get L1 icache sizes

This patch adds helpers to retrieve icache sizes, and renames the existing
helpers to make it clear that they are for dcache.

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191104023305.9581-4-alastair@au1.ibm.com