drivers: bbram: Introduce BBRAM shell

Add 'bbram' command which is very convenient tool for examining and
modyfying BBRAM content.

Examples:

Write one byte:
$ bbram write backup_regs 0x1 0xaa

Write many bytes (starting from 0x13):
$ bbram write backup_regs 0x13 0xa 0xb 0xc 0xd 0xe 0xf 0xaa 0xab 0xac

Read whole BBRAM:
$ bbram read backup_regs

Read many bytes (4 bytes starting from address 0x14 in this case):
$ bbram read backup_regs 0x14 4

Read one byte (from 0x14 address):
$ bbram read backup_regs 0x14

Signed-off-by: Patryk Duda <pdk@semihalf.com>