drivers/soc/litex: make 'litex_[set|get]_reg()' methods private

The 'litex_[set|get]_reg()' methods use the 'reg_size' parameter to
specify the width of the LiteX CSR (MMIO) register being accessed.

Since 'u64' is the widest data being supported, the value of 'reg_size'
MUST be between 1 and sizeof(u64), which SHOULD be checked at runtime
if these methods are publicly available for use by other LiteX device
drivers.

At the same time, none of the existing (or foreseeable) LiteX device
drivers have a need to access registers whose size is unknown during
compilation. As such, all LiteX device drivers should use fixed-width
accessor methods such as 'litex_[write|read][8|16|32|64]()'.

This patch renames 'litex_[set|get]_reg()' to '_litex_[set|get]_reg()',
indicating that they should NOT be directly called from outside of
the 'include/linux/litex.h' header file.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>