Add Kconfig symbol for L1 cache shift like the kernel does.
The value of CONFIG_SYS_CACHELINE_SIZE is not a reliable source
for ARCH_DMA_MINALIGN anymore, because it is optional on MIPS.
If CONFIG_SYS_CACHELINE_SIZE is not defined by a board, the
cache sizes are automatically detected and ARCH_DMA_MINALIGN
would be set to 128 Bytes.

The default value for CONFIG_MIPS_L1_CACHE_SHIFT is 5 which
corresponds to 32 Bytes. All current MIPS boards already used
that value. While on it, fix the Malta board to use a value of 6
like the kernel port does.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>